I'm still not totally sure if it's exactly a bullhead, but if it is it'll be just fine outdoors (they live wild in natural streams and other waterways that get plenty cold). Of course, it probably will eat any smaller goldfish.
It's legal to put your own aquarium fish into a pond (if not your own, then by permission of the pond owner), you just can't release them into a natural waterway. The concern is that the aquarium is a meeting ground for all kinds of funky diseases and parasites, due to the putting together of exotic/non-native fish species. If you release a fish that's been in an aquarium into your local water, those strange bugs could wreak havoc on an unprepared native system.
So I'd think putting them in your pond should be great, as long as there's no way it could escape into a nearby stream or anything. Of course, someone may come along and say No! That's a tropical catfish. So maybe keep checking back to see if anyone can give you a positive ID.
good luck!
