It doesnt matter what the species, environment or food availability, mantids will be mantids. I've never personally considered this particular species to be communal, even the communal 'gongylus' have their bad days, so the first thing for me to do is suggest you keep all nymphs seperate from now on to avoid catastrophe. The only time my mantids get to meet eachother is when its breeding time, simply because before that time, all other mantids could represent either a threat or food. The mantis that lost its arm could be ok as long as it succeeds in catching and eating food continually and if it does survive, it's arm should grow back. Other than that as i said above, try keeping the mantids seperate, they wouldnt remain in particularly close quarters in the wild, it's probably one of the main reasons most male mantids can fly.
Good luck with the mantids, great species to keep by the way, one of the few creatures that can do little/nothing, yet still be 100% interesting.
Dave