I'd love to take more in, but O'Malley was diagnosed with FIV - he was already in the house with Kiki when we found out so we figured that we wouldn't worry about it with her, but I'd be afraid to bring anyone else in. So far he's been pretty healthy though in spite of it.
I do know of a vet that gives quantity discounts, but you need 3 of them. I'll have to check with the vet we're currently seeing. I think I had called around with the last slew of kittens (had 3 and a cat at one time) and the only break was the 3 at a time one. I should check with the humane society - thanks for mentioning it.