Wow, they are pretty expensive roaches in my opinion. That's a pretty awesome friend you got there.
That is a good idea, feed them the hissing roaches and let the B. dubia colony stabilize.
I guess you can chop the adult hissers in half and then feed them if they're too large.
I had the same question, I don't know how hard the exo is from the hissers, but I assume the nymphs can be eaten fairly easily.