That really depends on what you want to do with the tank! Clean up crews can change subtly depending on if you're doing a reef system, or if you're keeping fish that tend to eat shrimp....or snails.....you get the idea. But to get you going, a good invertebrate "clean up crew" starter pack for a 55g tank would probably include 10 atlantic grazer Astraea snails, maybe 5 Cerith snails, a half-dozen large Nassarius snails (I don't care for the teeny ones, they just disappear) and probably 10 or so blue leg hermit crabs. Don't add any mexican turbo snails until the tank is more established or you see noticeable hair algae (and if you do, try to figure out what's causing it first) otherwise they may starve. When you DO go to add Mexican Turbos, probably no more than 5 or 6, and choose small ones. They'll get big, fast, but the smaller ones seem to be more industrious at keeping your rockwork algae-free. One or two fighitng conchs will also help the nassarius snails for sandbed maintenance.
If you're doing a reef tank and you don't have any fish that like an occasional shrimp cocktail, you could also add a pair of peppermint shrimp to the mix. They will help remove uneaten food, and they may also help if you have the occasional aptasia.
Some people like to establish a CUC before you start adding fish. I don't think that's necessary. If you have cycled your tank for a month, and you've QT'd a fish and are ready to add, go ahead and start adding your CUC as well. You don't need to add all at once, just pick and choose what you want to add over about a month's time for the invertebrates.
NOW, if you're talking advanced inverts like sea stars, that's a whole different ball o' wax.