My sub-adults eat anywhere from 4-15 med-large a day. I read that a roach is the equivalent of about 5-7 crickets. What I do is grab 10 and dust, then add more one at a time as needed.
i kinda gradually replaced the crickets. I'd feed only roaches a day or 2 during the week, rest the time crickets. Sometimes I fed a few roaches and topped them off with crickets. If I noticed my numbers started going down I'd back off a bit. I would experiment. You can always back off or add more if need be. As long as you have reproducing adults, your not going to ruin it, just delay a bit.