I have three large mantis, about 3", from TBS rock in a 10 gallon tank. They have only been in there about three weeks, but seem to be getting along all right. At least I haven't seen any corpses yet.
I made sure they each had their own "Cave" , 1/2 " PVC tube, and plenty of food.
There are several crabs in the tank also. The crabs are very skittish, but haven't been damaged yet.