Looks like a bluegill. I've never had problems getting sunnies to eat prepared foods. They are curious fish and will try to eat anything. He should be all right in the 55 short term; those temps are a little on the warm side, but not too high for him. These guys grow fast. They switch to a more fish-heavy diet once they reach a couple inches long, so keep a close watch; once he starts chasing your tetras it's time to take him out.
I'm sure you know better, BK, but just a general warning for others reading this thread: NEVER return fish to the wild after they have been in captivity. It's OK to release them into a private pond that has no connection to other waters (even during floods), but otherwise wild-caught fish must be re-homed or euthanized. There are several emergent diseases destroying populations of fish, amphibians, and other aquatics around the world; contamination from captive stocks is certainly one factor in the spread of these diseases.