Sounds like a good idea however it won't keep nassarius snails out but the crabs shouldn't be able to get in. Good luck. Here is a link to my thread about training a mandarin. Hth.
http://www.reefsanctuary.com/forums/saltwater-fish/36506-training-your-dragonette-onto-frozen-foods.html
I just picked up a skimz sm 201 it should fit in your sump its 9.5X9.5 these things are built like tanks and for the price I haven't seen anything close imo. A buddy of mine has a $2500 bubble king and a $1200 skimz and he said his skimz is running just about as well as his bubble king and at...
Probably not much you can do. Babies very sensitive to changes in water chemistry so dropping them into a tank that only had 50% of their original water probably shocked them.