I wouldn't be opposed to using a vet that gives all vaccines at once if I thought they were an excellent clinician. Most vets do too many at once. They rarely see problems with it. And there are a lot of dog owners who don't want to come to the vet every two weeks to do them one at a time. One concern is, "Will client be back for the other vaccines we think their dog should have." As far as I know, all of my dogs' vets would give several at once. I chose them for other reasons.. Vaccines are a negotiable subject. So for whoever is not comfortable getting them at once, you can always ask that they be split up.
AAHA is a good standard. When in doubt, yes, an AAHA hospital should be chosen. But not all AAHA vets or vet offices are good. So still reason to be careful..
I"m sorry about his GI upset.
Could be the drug, maybe not. If for whatever reason this treatment is still not agreeing with him, I would say it's probably time to find a new one. I can tell you this is treatment of choice here for an adult (who is thought to need treatment) with demodex and most all dogs do okay with it, but they are all individuals and what works for one might not work for another.
Not sure when Teddy's last bloodwork was done, but if it wasn't recent, it's probably time to do some.