You can keep early instar Chinese mantises together with minimal cannibalism if offered enough room and plenty of food. Around L3 though is when I would personally suggest separating them. You can sex them fairly easy at that age, so I would pick out a couple you like for keepers. You really need to over feed to help curb this species appetite and while you can get away with that for early instars, later ones will be at risk of shedding problems from being overly fat. Even if you are trying the communal thing, I would still suggest two cages as you should really switch them out as they shed so you only ever have one instar/size together. Maybe instead consider splitting the ten gallon down the middle with a barrier if you want to house two in it, and rehome the rest.
