Okay so for the trays - the rule of thumb is actually 1 tray per cat, plus one extra. With a variety of tray types (some enclosed, some open).
However, I have two trays (for 3 cats), one large one in the bathroom, and a second one for if one is being seperated (for one reason or another). This works well for me as my boys mostly toilet outside.

You will need to remove the urine from the area the peed, they have much more sensitive noses than us. Try vodka/gin or if your a bit boring, surgical spirit!
Innaporopriate peeing can be a sign of an underlying medical problem, so it may also be worth a vet check.

I have never had a problem with young kittens sharing trays, just make sure there is room for them to get in at the same time, otherwise it could get messy.

As for the litter, i'm not too sure, but i thought the only problem with kittens and clumping litter, was if they are born in it, they would um... clump to it.