I have a BP in a 75 gallon tank with an Oscar, and they are the best of friends. Sometimes they even sleep in the same pot. I saw them sharing a Beef heart cube the other day. Usually my Oscar charges the food, but not when the BP is there. They are great tankmates, because the Oscar spits out food everywhere when he eats, and the little BP goes around and eats all of the pieces. They were introduced to the tank, when they were both about 2" long. Now the Oscar is 9-10" and the BP is about 4-5". I did have a smaller firemouth in the tank, and it was constantly harrassing the BP.
If you had a larger tank, I would suggest severums, rainbow cichlids, other BPs or blue acaras.