Hey, not bad!  Good story, good prose.  There are just a couple of minor
details:

Ranma 1/2 Presents:
Shampoo 1/2

...

As the discussion was about to continue, a loud crash came from the front
door. Automatically Kasumi was on her feet first, concerned that if
something had been broken she would have to fix it. Habit at Kasumi's
taking charge of the situation made the others relax and allowed her to
examine things alone.

You tend to be redundant, especially near the beginning of the story.  In
the above paragraph, you might want to take out the word "automatically" and
reword the last sentence to something less awkward.

Suddenly Kasumi felt her heart race as more possibilities began to form.
Was she right? Was he Ranma? He did appear a bit young, but perhaps she
had been a but hasty in condemning all younger men. It was simply not
proper to stereotype an entire age group for being immature. Ranma
appeared very mature for his age, from what she had seen so far.

Heh.

I like how you write the first part of this story from Kasumi's point of
view.

Genma's reaction was to shift nervously as he cleared his throat and
looked towards the ceiling. "Well...," he trailed off.

Excellent characterization.  Still, it's only at the end of the story that
you point out that Shampoo feels a bit, err, degraded by being thought of as
a guy.

And with that Ryouga Hibiki jumped quite willingly into the fountain.

What?  No more P-Chan?  Heh, heh...

All in all, a good story.

-- Craig Putnam
Sailor Singularity