You know, its questionable where the Majin's came from nowadays. Toryiama is wishy washy on if Buu was made or just found by bibidi. I dunno where to tell him my opinion on a plot hole sealing backstory for them or not but heres my idea on that whole thing.

Majin (devil in japanese I think) are the servant class of the Demon Realm, serving under Dabora's race. Bam, Majin race origin that makes lots and lots of sense and patches up the majin buu loophole/makes GT fit into the timeline if you want to make it fit or not. lol.

Only throwing that one out there because that was my fav part of Xenoverse. Female Majin were soo cute.