some of your questions posted seem like something that is just up to you.
are you willing to start with glider that could have problems?
do you want to do introductions or not?
My wife has done about 8 introductions with all kinds of glider numbers. we had one colony up to 10 at one time. In all the introductions it seems like the gliders
really wanted to have new friends, and after a few spats in the tub ("NOT FIGHTS) they gliders were ALWAYS happy to curl up in a clean pouch together. the more gliders you put together, then it seems like the more chance one doesn't like another. In the end, with our experience, putting gliders together is about getting
them used to the others scent, so it is a familiar accepted scent, not a strange
invader scent. if you follow instructions, I believe there will be a very high
chance of success. (I hear there are a few gliders that don't except another.)
we had one rejection, putting 8 and 3 together, one of the 3 was chased at night and after 3 nights had to be removed.
age is up to you. If you get an unfriendly older glider it could be more difficult to get this glider bonded as they have spent so many years scared of humans.
we have had a couple that refused to completely brake being scared of us, but they are friendly enough to take a treat (then usually leave to eat in peace) picking them up usually gets a mild little bite just to let me know they aren't happy about being picked up. but ALL of our gliders have shown improvement in temperament as we have them and work with them.
forget about overgrooming and self mutilation as a lonely problem, take the glider to a GOOD glider vet, find out if there is a medical problem. but yes
gliders are colony animals, and seem to do great in a colony.