I don't know if this will help but we went through this not to long ago. Everyday we came home to mulitple piles of puke. I knew it was more than one kitty but I didn't know what was going on. Like you, some had hair balls and some didn't. I was freaking out and ready to haul everyone to the vet but hubby (the voice of reason) said that he thought we should wait. He thought it might be just a stomach bug and it would be over soon. Sure enough a couple of days and no more puke. I'm not saying that we did the right thing by not taking them to the vet but everything did turn out ok quickly. We had started warming up too when it happened and I think all the loose shedding hair they were ingesting was part of the problem. I gave them all a very good brushing and that seemed to help too.
So this is how it went in my house but you just never know with kitties! Anyway I hope your crew is over it soon because they are miserable and there is nothing worse than having to clean up puke all the time!! Keep us posted!