Either works, I'd advise wing beacon if the players are already in wing. The advantage of meeting in supercruise is both players can scan each other for weapons etc and make sure the "help" and "need help" claims are valid...
Limpets (see youtube link)
Comes from main tank, as far as I know extra tanks just add to the main tank as if you have upgraded it.
https://www.youtube.com/watch?v=nk0v0RbBLDA
Pretty much agree with this.
From personal experience, you may not have enough fuel to be confident to get back into SC, or enough fuel at all, so I think wings will be be the only practical method of meeting up. Not to mention how you would find someone in or out of SC in a system without a Nav Beacon - I don't think you are going to always arrive at the same place.