no, less light will not affect it at all. It is good that it will eat mysis. I also forgot to mention that you need to feed it a variety of foods. I like using silversides, krill mysis, etc. You can even get a lot of stuff from a grocery store.
Without knowing what species it is I can't tell you anything about size. maybe this is a fully grown mantis. Sometimes a mantis is left at an lfs for a looong time before someone comes to pick it up. lfs lights are usually pretty high so a deep water species might not tolerate it very well... that may be the case here, but again, i dunno until i know what species it is and where it lives in the wild.
any kind of fish food supplement will do. Just make sure it actually is a food supplement and not something like garlic extract that just gives food a better taste.
I believe it would try to molt and die in the process... i could be wrong though.
