barking is normal, not usually a health concern.
we have one glider that we got single and he hisses a ton when he wakes up and seems to pee. took him to the vet and the vet said he was healthy and active, plus he had no swelling or redness around his bottom, vet said he looked very normal.
Vet said the hissing maybe from a past infection and has become habit, said to keep an eye on his bottom side, but wouldn't give him medication just because he hisses.
If sneezing is in the sleeping pouch, that could be him grooming, hard to be sure without seeing or hearing him.
I look at how our gliders look, fur, nails, bottom side. How they act, each one has a personality and that personality does vary from time to time, but overall if they are acting normal, moving around at night normally. (NOTE: some sleep a lot at night. some are up guarding the castle.) Also, look at the eyes, gliders can get a tooth infection/absess and the bottom of eye will swell up large, then a vet visited is needed.