It is an interesting start...
I just had a horrendous idea: Harry + Neko-ken + Minerva's animagus form...
And way more funny: replace Minerva with a Nundu... I wonder how said monster would react ;-p
And if you want to push things, Harry is a Nundu animagus... With the neko-ken... Sometimes, I frighten myself... I don't think ANYONE would like to face a Nundu under the neko-ken dementia...
... But I just had a flash where said Nundu was on Kasumi's knees, puring, while she was scratching his head... Oh! My! God! So Cuuute! Imagine the wizarding world's reaction to seeing THAT ;-p
More seriously, I got the feeling that Genma realized that Harry is not Ranma... (and we KNOW that he would do this to his REAL son, too ;-p)
I hope you will not detail the training trip too much: I can't wait to know how Harry will react, being thrown into the mad-house that is Ranma's Nerima...
About the girls, I hope you will not even consider Akane:
First, with her insecurities and jealousy, there's now way in hell that she could accept being in a harem;
Second, I can't see Harry accepting her "I hit first if I have a doubt, then hit again if you try to explain yourself" on his person. (that and she would certainly NOT tolerate that he is better cook than she is...)
In fact, if you don't change any of the girls from the Ranma canon, I can't see one that Harry would take back to London and the Magical world:
Nabiki is intelligent enough, but way too manipulative;
Kasumi, I could see as a calming presence for him, but I can't see him bringing the gentle and defenceless girl into a war he is in the center of;
Kodachi... I can't see any reason to choose her: she is as insane as Bellatrix;
Shampoo could survive in a war alright, but as an Amazon "everything female coming near airen, I kill", she is absolutely unfit to be in a harem;
As for Ukyo, I can't see a mentally 16+ years old Harry not recognizing what happened and prevent the engagement, or at least prevent Genma from stealing the dowry, and thus giving her no reason to be in Nerima ever.
That's all for the moment!
Please, update soon!