very much like when I started my fish room some 20+ yrs ago.
How about building as staggered rack not flushed. Eventually I used nothing but 33G long tank on the racks some with permanent divers and some with temporary divers but all drilled at least two holes either on the back or bottom for draining purpose. Sure beats carrying buckets or using pump to drain. This was to use as acclimating/holding newly arrived.
As far as holding mated pair of species tank I used to used 10G, drilled also, onthe rack but side ways so I can stack more.
You might want to consider big air pump with either 1-1/4" to 2" PVC pipe for areation all around the rack above the highest tank to provide outlet for all tanks.
You can also centralize system hooled up but no need to be running on all the tank. You can have few local sump systems when needed.
Definitely apply some vapor barriers on the ceiling to protect the joists.
LMK if need some ideas.