I think crabs can be 50/50 in a tank in terms of likability! Small emeralds are cool - I've been lucky with mine because they've all come out and been active during the day, but I've heard for most people the emerald crabs only come out at night. When they get big they can also go after fish. I also have a pompom crab in my 10g, which I completely love, but the darn thing always hides and I rarely see him in even my tiny tank, can't imagine I'd ever find it in a larger tank. What about a pair of harlequin shrimp? You can get a few chocolate chip starfish to keep in your sump and just cut a leg off to feed to the shrimp and then it will grow its leg back.