i assume by "slimmer" you mean sKimmer... even though you said it twice. LOL
that being said, a fuge and a skimmer both are a VERY good idea for a mantis tank. i'd skip the canister filter.
it also depends on the type, species, and size of the mantis you plan on keeping. some very small species of smasher wont need much, but the larger peacocks, and larger stabbers will require heavy heavy skimming, as they are messy eaters, and often leave rotting food in their dens, or dump it outside in the sand bed.