crabs snails and shrimp with butterflies and angels?

I've maintained a 100G reef tank for years and plan on setting up a 30G tank for my daughter with a couple of dwarf angels (probably a bi-color angel and a flame angel or a coral beauty) and butterflies (probably a pair of small Heniochus Black & White Butterflyfish). I have some extra live rock that I plan to put in the tank, and I wanted to introduce snails and crabs as a cleanup crew - but then thought that perhaps the Angels and Butterflies might eat the snails, crabs and shrimp ... could anyone provide some advice about whether or not these inverts will be eaten by the fish that I plan on buying?

Thanks!

 

I know you don't want to hear this but...30gs is really to small for any of the mentioned fish. Lack of space can/will bring out aggression.

 

You should be fine with that. Dwarf angels and butterflies may pick at corals and sessile inverts (sponges, tube worms, etc) but usually don't bother with crabs, shrimp or snails as long as they're not small enough to swallow whole. IMOPE, they've never bothered anything as long as they were kept well-fed with a varied diet.