Bullheads can eat most any other type of fish that they can fit into their nasty, nasty mouths.
So let's say you were going to stock some 3" long fish. A lot of those 3" fish are going to get eaten by the bullheads.
Of course, there are some species of fish that may be able to avoid being eaten by bullheads better than other species.
Basically, the bullheads are another mouth to feed at the expense of you and the other 'desired' fish in the pond.
On the worm issue, there have been a few very informative threads about raising and keeping worms, but you'd have to search for them.
