Not sure why/how this went unanswered to start with.
Indeed even the most communal species usually listed, like the Ghost mantis (Phyllocrania paradoxa), can and do cannibalize (even with plenty of food/space/perches/hiding areas/etc.). If you are worried about losing a pet mantid the only sure action is to keep them housed separate.
That said if you are set on a communal setup and the occasional cannibalized mantid is not a concern then it is a option.
Also some species that are very cannibalistic can even be kept communal under certain circumstances with proper care. For example if the mantids were raised together as nymphs (same size/instar), and are of the same sex, some keepers have had luck under those circumstances when they are kept with plenty of food and space. Of course not all mantids will survive such a setup though.