If the fact that Feferi hadn't created the dream bubbles yet is a reason for players from other sessions not to exist in them, how are the dancestors there? Meenah learned about the bubbles and killed all the players in her session to send them there before the scratch that created Feferi's universe in the first place.

The dreambubbles have pretty much always existed but they were unstable and kinda spooky (see [S] Jade: Wake Up). Feferi convinced the horrorterrors to make special bubbles that the protagonists could use. Time works differently in the furthest ring so while Feferi had only just had the bubbles created they had already existed for years. This can be seen with Jadesprite talking about all the friends they had made and acting like they’d been in the bubbles for a long time even though the bubbles had only technically been created s few hours earlier. So having the Beforus Trolls in the special bubbles Feferi had set up is totally possible.