The water flow doesn't need to slow down. The habitats for the pulcher are brooks, and thomasi can handle what an aquarium offers and more. As long as they can get out of the current somewhere, they're good.
You are describing normal riverine aggression. If you had mbuna, it would be mayhem, but sparring is a way of life for Cichlids. As long as:
- no one is heading for a top corner;
- no one has chunks of fins missing;
- no one has flank or mouth wounds,
then they're just talking things over in their own language.
Some tricks? Each cave should look out to nothing much. A large rock placed near the entrance of a Pelvicachromis cave so that it blocks the view down the tank is ideal. They own what they can see, so obstruct the views. Plants can do it, but many tend to stalkiness down low and don't provide a barrier.
A thomasi come from slow moving forest streams, and savannah streams. Bear in mind that a slow stream is way livelier than 99% of home aquariums. If you put your hand in the tank and the water pulls at it and tried to move it, you're at the outer flow rate of slow. The fish are from high oxygen content water, so there is probably water movement nearby, especially considering the water there can be warm.
A friend who collected them said they were in several habitats, and that all were pretty normal, muddy water brooks and streams.