How big a tank do I need to consider having both fish and invertebrates cohabit nicely?
That depends on which species you want to keep. In general though, fish will eat shrimp, crayfish will eat fish, snails can become a problem if they overpopulate the tank.
I do have a heavily planted 10 gallon tank that houses cherry shrimp, a pair of orange dwarf crayfish, a dwarf puffer, and tons of snails. They seem to get along well enough. The dwarf puffer eats snails, it was a recent addition to control the ramshorns that have run amok. The MTS are still as numerous as ever, even the puffer can't get to those.
I have 2 craws, 1/2 blue&red & the other red.. when they get close they like to fight for while, but move on. I have a Dempsy, Oscar (2) & a Green terror. 2 pictus cats & danios. No problems on either side. Even though the big b&r craw goes when & where he wants, especially at feeding. Haven't even seen him go at any of the fish except to move them out of the way when he's on the move,
I have a 150 gal. tank. I might get 1-2 more (same size). kinda cool to watch them fight, no harm so far they kinda do a slap fight for a few seconds then there off. there pretty blind, so until they run into eachother they do there own thing.
I used to think that 55 was big enough, now I'm not so sure. I rescued a pleco this weekend, from the Petco. 4.5in of beautiful algae-eater. Unfortunately, it quickly came into the realm of my cray...and my cray started going on the hunt, i.e. chasing the pleco around the tank. Honestly, with the new fish in the tank already stressed out....Yeah, I moved the cray back to it's old tank, where she can be at peace in her own world. I'm not happy about it, but this morning my whole tank seems much more calm than I have seen it since I put the cray in the environment.