A pair should be ok by themselves. I say "should" because I have a "pair" that tend to beat the you-know-what out of each other the entire time they are not breeding. They are in a 55 gallon. Strange relationships, those chichlids...
While I know some will critique me for this tank size recommendation, I once had a breeding pair in a 29 gallon tank, with no other fish, and they got along great for years. A larger tank is, of course, preferred if you can afford it. Most recommendations I have seen tend to be around 40 gallons+ for a pair.