We have given them rabbit and guinea pig treats like yogurt items. I would say stick to foods formulated for mice and rats. Most treat packages have a picture of what animals they are for or say small animals. We have fed them Hartz Cockateil food because it looked like it had things in it that they would enjoy. Nothing bad happened, but I think maybe we should be more careful from now on.
In my experience, guinea pig & rabbit food has a lot of alfafa pellets in it and any mouse or rat I’ve owned hated alfalfa. So must of the rabbit/guinea pig food went uneaten and had to be thrown out. I would recommend rat/mouse diet mixes with seeds and lab blocks, but nothing with too much protein (not more than 14%).