Well they probably could if they made it to where in the World of MLP:FiM, Succubi & Incubi were not "SEX" Demons like their Real World Mythological Counterparts, but rather something else like maybe "CONQUEST" Demons, like they were Winged Humanoid Demons that live for Concurring entire Countries/Nations/Kingdoms, where they enslave inhabitants, plunder resources, and pilfer riches, after all when they brought in creatures like Timberwolves, Windigos, Changelings, Vampire Bats, a Chimera, Sea-Sirens, and a Bugbear, they made a bunch of changes from their Real-World(or at least Mythological) Counterparts so that they could better fit in with the franchise, so they probably could bring Succubi & Incubi into the franchise as long as they don't call them "Sex Demons" or let them have anything to do with Sex, because this is just too good of an idea for a villain/villains to completely ignore.