Since Japanese seems to be a popular language in space, and it seems that it is probably the native language of Jurai imported to Earth by Juraians who landed there secretly, partly Yosho, and partly some of his predecessors, it might be a good idea to import anime characters with Japanese names, at least for some of the more human looking ones that make up about half the population.
Rather than draw your entire cast from one other series, I might mix and match, and possibly mix some of them into the GXP universe in various ways.
For example, you could have Tennan Sakura (Converted Naruto character) from the same clan as Seriyo, who gets picked on for her large forehead, and has something of passive aggressive disorder bordering on multiple personalities, and is a super genius who will develop medical leanings. Sakura would become a Ranma fangirl after he protects her from being picked on and because Earth tends to produce interesting guys like Tenchi and Seina, and Ranma is even better looking than Seina.
And since Ranma is a gender changing shapeshifter perhaps he'd wind up in a 3rd set of dorms for neither/both genders, leading to the likes of Kashiwagi Makoto (Boku no Futatsu no Tsubasa) a naive and sheltered 'girl' who is actually from a hermaphrodite race with enhanced physical capabilities and telekinetic abilities that drain her stamina quickly, who is half-Juraian from a rich Juraian family but of no sort of nobility or tree possessing history, and is really tall and busty. And Shimeru Futaba (Futaba-kun Change!), from a race able to shift genders at will in adulthood, but during puberty they change when aroused and they grow up as the gender they were born in, Futaba was born male, and like many of his race, has some odd traits, particularly concerning the girl form at times, Ranma might briefly be mistaken for Futaba's race. ((Notably, in Futaba-kun change, Ryoko and Ayeka have cameos as students at Futaba's school, and in the end his family turns out to be descended from aliens))
And to throw in some additional fangirls who have become Ranma fans chasing him and stick a bit with the GXP themes:
Izumi Konata (Lucky Star), an extra petite girl who looks a bit like a kid, but unlike Neju, this is her natural adulthood, at least for her mother's race, her father is a human-looking type, possibly some sort of Juraian commoner (and noted photographer), she is an unrepentant otaku, but has also become something of an idol follower in terms of famous people in space, like Seina, Tenchi, and members of their harems, and is drawn to Ranma partly because he's from Earth and defeated Saffron, and partly because he is good looking in both genders and amusing to watch. She is also surprisingly athletic and good at martial arts. In particular, she is a big fan of cabbits, and might later manage to get her hands on one of the failed Fuku clones that don't have any special powers.
Aisha Clanclan (Outlaw Star) from the Ktarl Ktarl empire out of Jurai space, her people are a bit war-like and particularly value strength, and also run some successful pirating schemes in Jurai space, and her father is actually one of the leaders of the current Pirate guild, she was sent to infiltrate GP academy in order to get her hands on secret information and attempt to investigate Seina, as well as the new Earthling Ranma, and Earth in general, due to the threat Seina poses to Pirating in general. She initially just poses as a Ranma fangirl, but soon becomes a real fan who has real feelings for Ranma in her own way and personality. She serves a similar foil to Ranma as Erma/Ryoko, although without the multiple identities, merely secretly feeding intel to her father.
Nagisa Kaworu (NG Evangelion), the illegal result of a cloning project in an attempt to create an army of soldiers that would rival Ryoko by a pirate group, Kaworu was one of the only experiments to actually possess any special powers, he has a powerful telekinetic barrier around his body, a sort of personal shield, and actually generates a lot of energy, although most of it goes to this personal shield, which he has little control over. Kaworu has a creepy personality and unclear sexuality, and is Ranma's token fanboy, he also happens to be a bit suicidal and actually hates the GP and plans to destroy their headquarters with a powerful illegal high explosive invented by Dr. Clay and stolen by space pirates, his pursuit of Ranma has nothing to do with that, and he initially will try to make sure Ranma is away from the academy when he sets off the bomb, and also plans to be taken out with the station by sitting right next to the bomb as it goes off. The main reason he got into the GP despite his being a pirate experiment is because he is a masterful manipulator of people's emotions, and managed to trick them he is more sane than he really is.
Some of Ranma's teachers might be: Ebisu (Naruto) an egotistical teaching expert who was some minor teacher of a class Seina was in that Seina didn't really care about, who Seina barely remembers, but has a bit of an inflated ego because he'd taught Seina some minor thing personally in a single conversation he had with him, and thinks it is partly his own teaching that made Seina become a great hero, plus Ebisu has been a teacher long enough that he also played a minor role in teaching some other noted GP officers like Amane and Kiriko and Mihoshi, Amane doesn't actually even remember him, despite having been on the same teaching staff, partly since Seiryo kept hogging all the attention during staff meetings. Ebisu prides himself on being a polite gentleman, and is dreadfully embarrassed and paranoid about protecting his secret porn and sexy girls in magazine covers collection, a large part of the sexy clothed parts made up of Amane. Ebisu actually is a pretty good teacher, even if he is a bit forgettable and annoying, and truly cares about teaching and his students and is passionate about his work. Ebisu is made fun of by Ranma for being a closet pervert when Ranma catches how he eyes Ranma's girl form after a water accident and also discovers Ebisu's secret porn stash somehow, possibly due to the crazed 'Viral Seina Data' escaping into GP networks again and interacting with some 'viral Jyuusenkyo data' from Ranma that tries to turn all the AIs on the system female, and winds up shuffling a lot of data around, and causing 'Ebisu's secret stash, don't look, classified' to wind up on the console in Ranma, Futaba, and Makoto's room, which Makoto examines out of curiosity while Ranma and Futaba are present, partly pissing off Ranma as there is a security cam image of his girl form in a skimpy state of clothes in there, causing him to tease and pick on Ebisu as a sort of revenge.
Tanizaki Yukari (Azumanga Daioh), she isn't too bad as a teacher, but she turned out a bit too lazy for being a GP officer, and also people try to keep her away from spaceships, because while she hasn't managed to actually crash, she flies them or any vehicle she gets in, in an insane and dangerous manner and he vehicle always winds up rather beaten up, and anyone riding with her traumatized, they try to keep her as a teacher and living near where she teaches to keep her from driving anything. Yukari likes picking on Ebisu and is rather competitive with the other teachers, and hates Amane and Kiriko, who kept on hogging a lot of attention while they were teaching, despite also secretly having had crushes on them, as she is a closet lesbian. Yukari wants all the best students in her classes, and is usually lazy, but also gets fired up easily in any sort of competition or opportunity to show off, unfortunately she often messes up attempts to show off personally, so she likes to live vicariously through her more successful students and take credit for their work. She has, on occasion, used some limited hacking skills and her teacher position to modify the class rosters to get the best students, unfortunately for her, she also tends to get a lot of the weirdos and dunces as well, but she doesn't try to remove them from her classes, as she tends to form sentimental bonds towards all her students, although she is reluctant to show it or admit it.
"Health, Learning & Virtue will ensure your happiness; they will give you a quiet conscience, private esteem & public honor." Thomas Jefferson (1743 - 1826)
