You shouldn't feed them off yet. You should wait until your mixed nymphs grow to adults, then wait until their babies are nearly adults before you start feeding out of it heavily. If you don't wait, you'll just run out.
Once they are adults is when you can sex them, the males have full wings, the females have little stubs for wings. And as for choosing which to feed off... well you don't feed off females, and you don't wanna feed off too many males. I like feeding males that look like they've lost a fight with another one, or large nymphs... whichever presents itself to me first.
I use a heat mat underneath, but I know heat lamps on the side will work too, as long as you can make sure the temps are right, and you have a dark colored bin, or some other way to ensure the light doesn't bother them.
the cricket quencher is fine, but its horrendously overpriced. Check out gardening sections of like, Lowes or Home depot, or something (my local ones actually didn't have it, I had to go to Sears to find it) and you can probably find the dry water crystals. One bag of it is about $6 and will make about 50 gallons of wet water crystals.