i would think about upping the # of nassarius snails to maybe 5-10 as they primarily will take care of the sand bed. I don't know much about the emerald crabs you want but something is telling me that you may have territory issues with that many of those crabs in a 30. As far as a third fish remember the 1inch of fish / 5 gal
rule of thumb. your tank would be 6 in (not including the tail). Not to say your tank couldn't handle it but IMO
you may become a bit overstocked by adding a third fish to your order. HTH
65 gal reef
2 PC's 192W each , 20 gal sump/fuge
4 false clowns, 2 firefish, cardinal, midas blennie, toadstool, zoos, shrooms, trumpet coral, torch, pagoda, frogspawn, poylps, red open brain, devils hand, hermits, snails, 3 cleaner shrimp, coral banded and a emerald crab.