Try this link for the overflow size
.::AquariumPros.ca::. - Overflow and Drain Calculator
I have no idea about what pump I'd suggest for a system that size.
I would like to point out - What happens if there is a bacterial outbreak in a single tank? It could potentially wipe out every fish in the system.
It's a much more stable system to run airlines to sponge filters in each tank. You can then heat the room to whatever temperature the fish prefer. It is also way way way cheaper to do this.
Water changes are more difficult but in fry tanks the detritus needs to be siphoned out daily anyways.