some people get lucky and keep bettas in large numbers or in with gouramis...this IS a bad idea...heck, bettas often kill tetras just because they look similar to a beta. Gourami males and Betta males in the wild fight and kill eachother...it is how things work. They have had some of those instincts dilluted by imbreeding for colors by fish breeders, but the behavior still manifests itself.
So, no betta with the gourami. Also, it isn't so much the color that attracts betta's as it is the body shape. long fins and fish close to the bettas size are bad news. Even if I wasn't worried about your gourami and betta killing eachother, I would be worried about your Platy
...its the right size for the betta to consider it a threat.
I would hold off on any more fish till you get them in your bigger tank. at that time look into adding a couple more ottos, and maybe a small school of tetras. Make sure when you get the tetras do not get small ones such as neons or ember tetras, make sure they are lemon tetras are or larger (over 1.25" and full bodied). Smaller tetras could get easilly bullied and stressed by your gourami. Also, get a full school of them (6 or more) otherwise they will be timid...in a school of 6 or more lemons don't get bullied. I have had bettas and gouramis at differant times in my community tank, and the lemons are the bosses,lol.