I'd wait until all the joeys are wean or mom will be extra protective and probably try to fight.
I actually find putting the cages next to each other a bad idea, it kind of makes them look at each other as rival colonies AND if you do do it anyway (some people say do it, some people don't) then don't make them touching. Gliders can still fight through cages and Kazko had a glider get a tail bitten off through a cage.
Honestly NOBODY can tell you how long it will take for them to get along, it has taken some people one introductin and they can put them together, but more commonly it takes a few months.
When you do go to introduce them you should do it one glider at a time or they'll gang up on your new guy, and don't just dump them in a cage do it in a bathroom or something and wear gloves to seperate a fight
Also, be realistic and know that some introductions don't work and will never work.
I would also let your new guy settle in for about a week at least before you start doing any intros if all your joeys are weaned.