The general consensus is "just dont do it"; however, over the past year and a half that Ive kept mantis shrimp I have kept them with a fish buddy 90% of the time. In my opinion you have a few things to consider: the type of mantis (smasher or spearer obviously is a major concern, but Im talking more abou the species of smasher), amount of water, type of fish (where they hang out, are the inquisative fish etc). I have had great luck keeping a true perc and a gonadactyus smithii togeter (10 gallon tank), I attribute this to: the smithii not being that aggressive of a mantis, alot of show though, and most clowns will hang out mid-top of the tank.
When you consider adding corals you should also be fine, unless of coarse your speaking of large stony corals, which at some point become a nice place for a mantis to bust a cave into, which is how some species form caves in the wild. So I guess what Im saying is: fish+mantis-think it through and cover some "what if's", mantis+coral-most types will be fine (just matters what you care more for-your mantis or your coral).
Cheers...it's Friday!!
Brian