First of all, thank you for giving this baby a home...
That being said, as the mom of a few strays myself (only one at a time, which makes me the minority on this site!), I have to agree with previous sentiments - get the kitty to a vet! ASAP!
You can not tell what the potty issue is, but I also agree that keeping inside - especially at this age - is the best thing. I use a standard litter, Tidy Cat, which is not clumping and does not have any special deodorant in it. My vet has always recommended that route, and I've never had trouble with smells.
The basic explanation on the potty issue may be worming. This is a simple pill treatment and could be just the ticket.
Please keep the kitty inside if possible, and GOOD LUCK!

Pinot's Mom aka Maggie