definitely give him a litter tray, preferably where the puppy can not get to. Use the same brand of litter as the rescue, and ideally bring a scoop of it home with you to put into the clean litter. There may be toileting accidents, but not often. You should keep a new cat indoors for at least six weeks when it is coming to a new home where it does not know the humans either. Especially with a puppy in the house, if he has not bonded to you he might decide he can do better elsewhere and do a runner if you let him out any sooner ...