Tank Volume to include Feeders

[Scouse]

Am wondering at mo at a few possibilities of set up, either...
Bimac or Native UK Octo, Just gotta decide on tank set up.

Decided I need either two or three tanks

Main Tank
Shrimp Tank
Crab Tank

Question being can I combine the shrimp and the crab tank? Anyone ever done this or any ideas on set up

So then once decided if I could do a multi-feeder tank could I combine it is on the same system as the octo tank. I know it would not be best to do this due to stability as if some feeders die then they could mess it up (plus crabs being messy). But am wondering if someone may have done this and what water volume are we talking about to acheive stability without quoting the ocean

BTW wouldnt the shrimps eat some mess i.e. the left over flakes from the crab food anyway

cheers