Betta and Honey Sunset Gourami?/ Overstocking?

[King of Candlesticks]

So recently two fish have completely and utterly disappeared from my 12 gallon fresh water tank. (trust me I've looked everywhere). I lost a Corydoras Catfish and a Red Wag Platy(Left by himself after my other Platy died a while back).

Here's what I have in there now:

1 Honey Sunset Gourami
2 Black Mollies
2 Corydoras Catfish
3 Fancy Guppies

I couldn't put my old/ a different Betta in that tank right? Because of the Gourami and the Guppies?

I'm going to replace the catfish but I was looking for a solitary fish I could add with out overstocking the tank.

Would a female Betta work? How about other solitary fish any recommendations?

Thanks

[Kim]

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!