I have put ghost shrimp in my tanks in the past, and they are so fun to watch! No, you don't have to heat the tank. The LFS
I used to buy them from keeps their ghost shrimp in tubs with the koi they have for sale. They don't get very big, I'd say about an inch or so. Some fish will eat them, but I had mine in a community tank with gouramis, bettas and corys, and no one bothered them.