I've kept sunfish and rock bass, they make great aquarium fish. Yours looks like a smallmouth or possibly a largemouth bass; they are very similar when very young. If it is, it will grow a lot larger than any sunfish. If you decide to keep it and it IS a bass then be ready for it to grow fast and large...like 18-24 inches!
I too am mystified as to how this fish would be in with feeder goldfish.
This is a very interesting thread- please keep us posted and good luck!