The smaller the tank, the bigger the risk. Too big, IMO.
What size is your tank, Pimpala?
I've had all my mantis' eventually end up eating their fish mates. Tanks all 25g a below, besides a split 40g. Ya, nothing like the ciliata, who went all ninja on the damsel. I actually felt too bad to do it a second time and I'm not usually soft like that :-/