Krystalak and Obsidius

It depends on how much you're willing to change their backstories. Krystalak is made from space godzilla's crystals so according to his back story that's the only movie he'd work in, but if you just said he was some king of radiated monster he could fit in any of the showa movies, and just become a background monster on monster island. In my opinion Obsidius would have fit in Godzilla vs megalon instead of Gigan. The seatopians(or whatever they're called) just call up into space to get Gigan, when it would have made more sense if they just sent up 2 ground monsters that they controlled since there's no mention of them in the prior movie with space space hunter nebula, and doesn't make sense how humans who live right above them don't know who they are but space aliens do