What for?
If you have no algae for them to eat, why would you be in a rush to get a bunch of algae eaters?
Relax. No rush. I'm _not_ a fan of buying a huge number of snails and crabs in some 'clean up crew' package. In my experience, the vast majority of such packages end up dead within a few months, which does nothing to help 'clean up' your tank. IMHO, you're much better off selecting individual or small groups of animals for a specific task, when and if you find your tank in a position to need their services. Got some hair algae? You might get some Trochus, Astrea, or Turbo snails... or you might get a rabbitfish or tang... depends on your tank, your preferences, and your needs. Want to help break up detritus in your sand? Pick up a handful of Nassarius, or maybe a sand tiger conch, sand sifting starfish... There's a variety of critters that might help.
My point is, rely on your tank to tell you when to add clean up crew, how many, and what kinds... and use your own judgement and preferences to determine what you'll add... not some 'sold by the gallon' clean up crew. I have no way to back this up, but I've long suspected that some of the diseases that plague our systems come in from these wild collected, cheap, high volume clean up critters.
Oh, and I'm real cautious about adding crabs to a reef. I have a few small hermits... and I'm always looking for the next one that's made himself at home in the middle of one of my LPS or Soft coral colonies. IMHO, all hermits are opportunistic feeders, and may one day decide that your prize Hammer coral tastes better than hair algae. I do like a big old rock crab, gorilla crab, or overgrown emerald crab as a sump monster, but aside from that, I'm real cautious with crabs.
