Rimuru is BOTH genders because he can create genetils as seen after he got his body. Of course, it prob won't be that easy, but if he can destroy universes with knull, he can 100% become a boy or girl.
A being that can wipe out many universes & make more . . . Yeah, Cid, don't even try, for all we know he's probably the one that put you there, luckily these stories have absolutely nothing to do with each other