Sure. But there are a ton of other options as well.
Be careful with Yellow Labs, however. They tend to be one of the most docile Mbunas. You might want to add them first and let them get situated before introducing the others.
You could probably even add more to a 55. For Mbuna, the more there are, the less aggression there tends to be. Just be sure to provide plenty of hiding spots for them. Mbuna come from rocky areas.