You put them in hydroton when they are hardened, and have about 3 inches of good root mass. If they are not hardened, you can put a baggie over each on for a humidity dome, and harden individually, but that's the more dangerous and definitely more difficult way. Both work.

About 1/4 to 1/3 strength nutes at first, every week or two increase until full strength.

A few light leaks are OK. It doesn't have to be 100% light proof, but it should be mostly. I'd say 95% light proof is fine.

I would guess several plants in one big container of hydroton should be fine, just give them their space. I did it in dirt once and everything was fine.