I currently have 10 cats and have tried everything. I swear by worlds best cat litter-it's a little pricey, but worth the cost. I find it clumps well, is odorless, absorbs well and lasts 2-4 weeks (with daily scooping). It also doesn't impact the environment or landfills the way clay litter does.
I live in New Hampshire and have the same worry about a couple ferals Ive been feeding for a few years. In live in a city and usually Muffin and Gandolf come around every morning and night like clockwork for their meals. I feed both wet/canned and dry (I use SnuggleSafe discs under food dishes...