What type of fishes can I add on into my almost 2-inch bullfrog's tank?

You're unlikely to find any fish that your bullfrog won't eat, and/or will live without a filter. Your best bet would be to get a small submersible filter (roughly $15) for his tank, just for sake of good water quality and ridding yourself of mosquitoes, if he isn't eating them-- And forget about adding other fish. Bullfrogs are notorious for eating fish, which is why they are an unwelcome addition to our country, and illegal to release into the wild in most states.