I have four that are about to go into the DT.
They have established their pecking order and hierarchy. They're still all females, for now, and all alive, for now. Luckily mine eat everything: FLAKE, mysis, cyclops, pods...
Three constantly swim mid level and one is always on the bottom. The fear of the one at the bottom dying worries me, and I would be afraid to add more. So your question has popped up in my mind too (i.e. adding more later, not the regions part).