You don't need a single crab, snail, shrimp, or starfish to have a perfectly clean tank. The term "Clean up crew" was simply invented as a way to sell livestock. On the other hand, if you like crabs and shrimp and such, then add one or two because you like to look at them, and not because you think they are going to keep your tank clean.
Remember, every piece of food and additive you put in the tank must be removed somehow by you. Water changes, filtration, and reasonable feeding are what keeps a tank clean. Snails, crabs, and shrimp poop just like fish do, so they add to the bio load.
Whatever you add to the tank must have food to sustain it. They say you must "recharge" your cleanup crew every so often, but this is simply because they end up starving to death, and/or killing each other.