I'd avoid having everyone meet everyone, or rehashing the book plots too closely. Either, through some magical mishap, a small group of characters from one world end up in the other, and have to find their way back, or Nerima is, and always was, somewhere on the Discworld.
In the first option, three or four is enough. Much more than that, and most of the characters will just get wasted. For the second, the obvious place for Nerima is the Counterweight Continent, but that's away from the main action, apart from Interesting Times. You could make it a suburb of Ankh-Morpork, but that might be a bit too close to the main action for fitting the backstories together. I'd make it an eccentric small town, within a week's travel of Ankh. The Jusenkyo area would be in the Hub, where all the mysterious monasteries are, and the strongest natural magic.
The Discworld characters come in natural groups, any of whom could be dropped in canon Nerima. Going the other way, either just send the characters you want to pair up, or pick ones who will clash, not necessarily a usual grouping. Imagine Ranma, Cologne, and Konatsu, for example, or leave Ranma at home and send Akane, Mousse, and Genma.
Granny Weatherwax doesn't lose, and she never turns down a challenge. If Ranma characters get dropped into Lancre, they'll stay there - Granny would never go running to the wizards for help. Granny also understands stories well, and she doesn't like them. She'd deliberately break the tyranny of status quo - curing the curse and Ranma's cat phobia, while Nanny Ogg gave him a few lessons on romance and sex. Unfortunately, Granny thinks she can cook. If she tries teaching Akane her culinary secrets, the results would certainly be interesting, but not edible.
Drop the witches in Nerima, and they'll still meddle with everything, trying to break the status quo. Cologne and Granny Weatherwax would probably go in for icy politeness - they've got different styles, and little respect for foreigners, but they're too mature to rush into a fight.
Drop the Nerimeans anywhere else on the disc and they'll end up going to Ankh-Morpork, where the wizards will be able to send them home. The locals won't be actively trying to sovle their problems - apart perhaps from the wizards trying to break the Jusenkyo curse -but things will get shaken up anyway.
On the disc, any encounter with Rincewind is likely to be fleeting. In Nerima, he'd have a lot of lucky escapes.
Vimes would apply common sense and cynicism in equal measure, whatever the circumstances. On his home territory, he'd consider the Nerimeans a nuisance, and give them sensible adive in a moment of exasperation. In Nerima, he'd try and keep the watch unentangled in local affairs, for pragmatic reasons, though Carrrot's idealism would cause problems, and he'd react badly to Kuno.
The wizards have appetites even bigger than Ranma's. They'd view him as an amusing curiosity, which would rankle. If they were visiting Nerima, they'd cause problems everywhere, meddling with the technology they don't understand, and they wouldn't listen to any warnings. On the Discworld, they'd be a bit less annoying, but there'd be quite a bit of dangerous magical experimentation, with Ranma as test subject. Remember, their first proposal for an hangover cure would have left the patient liquefied.
Martial arts are rare on the disc, outside the hub, though Cohen should be able to beat Ranma comfortably. Some of the non-human creatures mightl be a challenge. A troll is a lot harder to hurt than Ryoga. Ranma won't do so well against magic. he can dodge fireballs, but not curses.