nice to hear about another mantis lover.
12 mantids will almost definitely not coexist. If you wan't to have coexisting mantids make sure you have lots and lots of holes in your LR (wennerae chill in holes in LR, not sand) of DIFFERING size. The only problem i see you having is the fact that they are wennerae and are one of the more aggressive mantids. Don't expect great results, but in a 400gal with lots of holey LR i expect a few will coexist quite nicely.
Do you mind posting a pic of the green mantis you got in the 2gal? How big is the mantis? No fully grown adult species i know of are recommended in a 2gal.