I tried to answer this in another thread this evening, but let me have another go.
This time, let's assume a well run in aquarium with adequate canister filtration and perhaps, in systems over 20 gal, skimming.
Animals under 1 inch total length can do very well in 1/2 gal tanks or even smaller.
For a 2 inch gonodactylid, I like to have a 1-2 gal system.
For a 3 inch Pseudosquilla or Gonodactylus, 5 gal will do nicely.
For a large G. chiragra or G. ternatensis (3.5 inches), try for a 10 gal.
For a 4 -5 inch O.scyllarus, 20 gal.
For a big (6+ inch) O. scyllarus or Lysiosquillina, I like something larger than a 20 gal.
Dividing tanks certainly will work and can have benefits. We keep 5 large O. scyllarus or two Hemisquilla in each of our 100 gal systems. Probably because of the stability of the systems, they generally do better than individuals in 20 gal systems.
Roy