Sick Possible URI or is he fine?

I'd take him in. It's easier to treat a URI earlier than waiting until it progresses into something more. I wouldn't give him antibiotics without having a vet check him because guinea pigs can build up a tolerance to antibiotics, just like humans can, so you don't want to medicate unnecessarily.

That is pretty much exactly what happened to my Clover. They vet says that she might have an infection in her mouth, and prescribed 0.2 mL of Enrofloxacin (Baytril I think) 2 times per day for 10 days. Is this OK, too much, too long etc? I thought it seemed like a lot for a few sneezes and a runny nose, and I also thought there was a milder version of the medicine for younger piggies (Clover is about 4 or 5 months old) that wouldn’t risk stunting her growth...

By the way this is the vet we saw:
(broken link removed)

Baytril is probably fine at 4-5 months, so I wouldn't worry.

We don't know whether that's too much medicine or not. You have to know how strong the medicine is (how many milligrams per milliliter) and how much your pig weighs to be able to figure that out. But assuming a normal strength for the enrofloxacin, that sounds about right.

You need to watch to be sure she continues to eat, as the med can affect her appetite. Get a probiotic (bene-bac, or plain human acidophilus or kyodophilus) and give it to her about an hour and a half after each dose of the medication.