Your oldest has the greatest likelihood of being the one...however don't discount the others.
I would use Revoloution on all the pets at the same time. It works for worms, fleas and ticks. The only worm it doesn't work on is a tapeworm and if none of them have an unusually greedy appetite it is generally safe to assume they don't have that kind of parasite. You do have to get it from the vet. The other vet prescribed med you put on the back of their neck doesn't work on worms so make sure it is Revoloution.
If there continues to be pink tinged barf, try to figure out which one it is and take them to the vet for tests. Our shorthaired orange tabby just died (yesterday) from a cancer that went into his stomach & upper intestines. It took us a while to figure which of the six cats was doing the bloody barf because we had to witness who actually did the barfing. Sometimes, certain pet foods can irritate a cats stomach too depending on the cat. Whenever a brand of pet food changes their formula we tend to know because it either changes the cats stool or their stomachs don't like the change. Trial and error method. GL
Ugh...that's tough, it depends on which parasite it is. Look out for weight loss and fur loss. I'd worm the lot of them just to be safe.
Otherwise, I'm sure if it wasn't a one time thing the culprit will step forward and upchuck on your shoes or something similar as a form of confession. :)
Oh, and do worm them all, some parasites are very contagious.