Add your clean up crew once all your numbers (ammonia, nitrate, nitrite) are stable--ammonia and nitrite should be zero, and nitrate low. Essentially, hold off until the tank has finished cycling. I think that might be a bit much to start with. I always recommend people tailoring their clean up crew to the specific problems their tank has--each snail/crab is good for a different type of problem. I would add things as problems arise to combat--and I always recommend steering clear of emeralds unless it's a last resort on bubble algae. Some people have very good luck with them, some of us have had bad luck, I'd never say absolutely don't get one, but adding 2 at the beginning when you don't even know if you need them to me seems unnecessary. Do a search of this forum, there have been a lot of good posts on suggestions for clean up crews as well as one thread a while back detailing each type of snail common in aquaria with the low down on their pros and cons. Just found it--I've bumped it for you, it's in the Reef Tanks forum by kipass entitled 'my take on various snails'
-leigh