Julien,
We always recommend at least using the latest of our software as that's what we know best.. and it also has all the best bug-fixes in it.
Regarding jSebold, we don't directly integrate with that, but we do know that JFBConnect has worked well with it for many users. I've never used jSebold, so some of these questions may be off/sound wrong.. Obviously, jSebold seems to extend the user table in some way. JFBConnect creates standard Joomla users, so there shouldn't be any need for integration as long as jSebold properly uses User plugins to detect new users and create whatever is necessary in their system. That's why we recommend the newest jSebold version... it may have more support for 'outside' user creation.
Please let us know if the above helps or makes sense. If you can explain more about the page within jSebold that you're going to, that may help. Also, if there's a way to see users in the Admin area of jSebold, make sure that any user created through JFBConnect is properly appearing there.
Thanks,
Alex