A wilder possibility, why have Nabiki at 17? If she's a freshman (15 years old) at Mugen Academy, and is tutoring at the Junior High to make ends meet, she could meet Makoto. That puts Ranma several years in the future. The reason for Nabiki to be there is a secret she won't share, or only slowly unravelled, e.g. she is basically there as part of 'creative sentencing' for getting caught doing something (details left to the author, or never revealed); or she just got Akane's and Kasumi's dander up and they threw her out but still support her. The people at both schools tend to lump the two 'juvenile delinquents' together, because nobody else wants to deal with either of them. Remember cleanliness and purity are important in Japan, and both are seen as tainted by their previous bad behavior.
Let them grow on each other. Nabiki mooching food off Makoto, in return for tutoring or the occasional goodie out of Makoto's price range. 'Hey I scored a couple of real steaks, I'll let you cook'em up and we'll share.' Also as a shock to Makoto, let her see Nabiki take out a few thugs. Weak in Nerima may still make you a martial arts god in Juuban. Their relationship doesn't have to be romantic, but a definite kohai/sempai tone. Makoto needs a friend, and Nabiki needs to know not everything valuable can be brought or stolen.
I'd also suggest you stick with the early Nabiki characterization, less vicious and a little boy crazy, that gives her something else in common with Makoto.
And if Nabiki is going to the Mugen Academy, she could also know Hotaru, and the other Outers, but not as Senshi. Unless her skill at uncovering secrets comes into play. Of course her immunity to weird may make her a possible 'big sister' to Hotaru, after Makoto has softened her up a little.




