On Mon, 26 Aug 1996, Richard Lawson wrote:

Shampoo and Mousse, on the other hand, I can't explain :)

Actually, Ross again offers one good explanation - Mousse finds flying
to be useful.  And nowhere in the manga do we see an indication that
Shampoo is unhappy with being a cat.  In fact, she seems to enjoy it
once a while.  So maybe they could if they wanted to; they just choose
not to.

Well what about the Ranma Movie #1 at the end... Soun says  "Saotome, 
don't you think we should have stopped by the cursed springs of Jushenkyo 
while we were in China?" and then everyone including Shampoo and Mousse 
scream.