I have a little more time to post and think today
, for some reason I was thinking your males were older than 4 or 5. 4 and 5 yr olds really aren't that old, old for gliders is 8 or 9 yrs old and up.
Usually around 8 or 9 yrs old is when we really have to consider if neutering is best or not at that point in their life.
One of my males was 4 yrs old when he was neutered just before we adopted him and all was fine, he was back to his normal self that same day.
If the gliders are healthy there isn't really any higher risks for a 4 or 5 yr old than a neutering a younger glider would have. A lot of breeding males get retired around 6 to 7 yr old, some are neutered some are left in tact it's really up to the owner and vets discretion.
Your vet may have agreed about the age due to lack of experience with neutering gliders. I would ask him some more questions like how many neuters has the vet done, what methods do they use ( there are different neutering methods ) traditional pom on, traditional pom off, lazer pom off etc... does the vet give pain meds or use e collars for after the procedure, stitches or glue or cauterized for the neutering site?
what is the after care needed?
If the vet can't answer any of this to your satisfaction look for another vet that has more glider experience. I have resources all over the country if you give me your city and state i can maybe help find you one.
Their smell will decrease considerably if you get them neutered. Try not to over clean their cage that will just make them mark things to stink them up even more.
As for best potential new gliders, there really is no clear cut answer to that. Even under the most ideal situations new glider additions are a 50 50 kind of deal it's either going to work, or not work.
If your males remain intact I would probably keep them as a pair. 1 if you go with a female they will breed and it may cause issues between the two boys they can and do fight for breeding rights, 2 if you go with a neutered male the potential for them not getting along is pretty high. They get along now because they have always been together, adding another male or female is going to change the dynamic in every possible way.
If you decide to neuter, then either gender can be introduced to them, which is still no guarantee it will work long term.
These are merely my opinions. I have done several introductions in different combinations with 6 of my 9 gliders. Some were successful and some were so unsuccessful that one of my females suffered facial injuries. She has since recovered and is doing fine. Introductions with any combination of age and gender is a crap shoot, we just can't predict the out come all we can do is do our best to listen to the gliders and prepare them to the best of our abilities for the introductions.
It can be hard, stressful, scary to do intro's especially the first time. Research different methods and get to know the gliders, give it some time before you make a decision. Once you do decide then gather up what your going to need, a second cage and set up, vetericyn vf, emergency kit.
I sent you a pm with friends facebook info, your also welcome to add me if you want
https://www.facebook.com/carla.sloan.1