First of all, you have a mantis with other inhabitants?!!! Thats a HUGE risk you are taking. Mantis shrimp are pound of rpound one of the fiercest creatures in the ocean. They can and eventually will eat everything in your tank. Many people do keep mantis shrimp, but they are usually kept by themselves.
As for the cleaner shrimp in the 120, there is a chance that the puffer or eel may eat it. There are tons of cases where these more agressive fish will leave cleaner shrimp alone and will even let the shrimp clean parasites off of them. But that really depends on the temperment of the fish.
With the 55, I think that would be a great place for the cleaner shrimp if the mantis werent in there.