In general, I agree, not a great idea.
But I did actually set up a few of these once, because, like it seems you have, I had more than I could even begin to count. And it's a more humane way of thinning them than just arbitrarily picking up a few of the little guys and dumping them in the freezer
So if you absolutely want to try it, here's what I did:
10 gallon aquarium, fine window screening as a lid, plus a whole mess of cheap walmart silk plants, preferably the ivy kind that you can sort of bend, and comes with a lot of foliage. If you literally cram the tank full of these, so that theres barely any space to move around, you can set about 100 M. paykullii loose, and then just dump in a few 100 fruit flies every other day (use D. hydei as they age, then bottle flies)... you might end up with 20 late instar nymphs. I made two of these, and ended up with maybe 30 subadult/adults, mostly females (surprise, surprise).
If you mix hatches/sizes you will lose just about all of them. if you use siblings and just let the ooths hatch into the terrarium, for some reason that seems to work better. I think it's because they are all the same size, all eat at the same time, all molt together, etc. so they're always busy doing their thing, and don't interact too much. And flies are the best, because the Miomantis will always go up, to the lid, and so will the flies, so they stay full longer.
I think the other trick is, keep them at room temperature, or even cooler. They eat less and don't sem to attack as often.
Good luck!