Judging by the comment of "shrimp would definetly be too easy for a mantis so scratch them from the list. I would also prefer thicker shelled crustaceans too but I won't be too picky either.", I assume you are going to be keeping the mantis in that tank?
With it in the tank, its probably going to kill anything it can, including your damsels and any other fish it can catch. Even thicker shelled snails or hermit crabs will get eaten if it wants, if the mantis is too small to crack the shell, then its likely small enough to enter it when the critter 'retracts'.
Your bioload and fish will also affect what you can/cant keep in the tank for cleanup crew. If you have a bunch of tanks, I'd have a very minimal cleanup crew as the main cause of death in captivity for hermit crabs at least is starvation, and I am sure the same would go for snails (minus being easier food targets, even for hermits). If you plan to put a trigger in there, alot of cleanup crew is out of the question as well. More or less rock will also have an effect on how efficiently the waste from the cleanup crew can be processed, and whether or not there is enough area for food for them in the tank. The goal of a cleanup crew is not to 'get rid of the algae', or 'eat the leftover food', but to break the nutrients(algae, leftover foods, etc..) into a form that is more easily manageable by the bacteria in the system. This also requires those nutrients to be taken out of the system via filtration (skimmer, etc..) and/or frequent water changes.
A list of planned fish + live rock amount, as well as more specs such as filtration will help us to help you
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Chris