Could the bodies have gotten stuck in any decorations where you can't see them? Maybe they died and the other fish ate them??
Yeah, the male betta is a no because the gourami and the guppies would provoke aggression. Some females can be quite aggressive as well, and may fight with these fish as well. Probably not the best solution.
Sorry, I don't really know any other solitary fish that you could add. Maybe someone else would know though. Good luck!