Get the kitten to the vets as soon as possible to get it checked over, they can advise on food depending on how old they think it is, and may have a home for it. The ranger will also take it if you ask.. ( I didn't think RSPCA destroyed cats any more.. )def not kittens as they are so easy to rehome, its old cats that are the issue.
If you keep it, keep it shut in one room with its own litter tray ect, until it cans be vaxed ect and introduced gently to your other cat.