It's the same way uncooked pork can give people parasites; a lot of wild frogs carry parasites, and they can be transferred to the snakes. But since you caught yours, he's already been exposed to these things, so frogs shouldn't hurt until you can get him on other foods.

With fish, try to stay away from anything with red or yellow coloration. That usually means the fish contains Thiaminase, which can cause neurological problems if they get too much of it. Can you get salmon or tilapia from your grocery store? Also, guppies are safe to feed.

If it won't take scented pinkies, try chopping up the mouse with chopped bits of a food they will eat. Eventually they should start taking just the mouse with nothing else.