I have one of these that I keep a juvi RES in (turtle). For one, you could keep a female festae in this. We would just call it cruel. I wouldn't even keep a festae (male or female is moot since it is too small for both) in a normal 55g. Again, 3 RPB is possible... if you don't want to it to swim much when its full grown.
Set up a breeding tank for some angels, 50g will hold a good few and they appreciate vertical height as much as horizontal length.