First, it's very hard to tell how an individual glider will take to other gliders at any age or circumstance. However,mi personally have successesfuly introduced a lone male to a pair of females. The male had already been neutered, he was 4 yrs old, and had lost a cagemate. I'm not sure how long they were together, but couldn't have been too long. His mom before me told me that she got him from someone who rescued him and it was hard finding a glider he'd get along with. She finally found success with an older female, but then eventually she past. I do not know how long he'd been alone throughout his life, or even for sure how many homes he had, I know at least I provided his 4th.
Anyway, what I'm saying is it's possible, he may love having glider cage mates, but there is never a garauntee, it's something we have to take chances with, and figure it out.
Good luck! Oh, and don't beat yourself up, you're doing good now, and evidently, if you've had him as long as 6 yrs, you couldn't have done too badly.