In my propagation tank I had a mix of roughly 80 rainbows, black widows and a few others without issues (about 50 BW and mixed others). What you hear about re: chemical warfare tends to boil down to a couple of things in my opinion (with some experience behind it.)
First, I believe that in 95% of the cases where people scream "all my Nems dies because of chemical warfare" it's because their tanks sucked in one way or the other for Nems. They will tell you everythng was fine for months, but the thing with Nems is they tend to die really slowly under bad conditions. They are actually quite hardy, but if things are bad (light, stability, water quality) they will slowly die over many months. Many people drop multiple Nems into a tank that's not old enough and then blame anything but themselves.
Second, and I never even believed this until I propagated for a while, there MAY be something to mixing wild nems with aqua-cultured nems, but no one has ever shown it's chemical warfare. It could also be wild bacteria, viruses, or most anything.
In any case, you're mixing types that have been aqua-cultured for thousands of generations in home tanks, and it's just fine.