Hi there. Welcome toe the Betta appreciation society lol. Most Bettas are kept with little to no fish in their environment. They are aggressive and territorial. I have kept small fish with them, like Pygmy Corey's and Norman lamp eyes. But he picked them off one by one. Large corydoras seem to work and a 60 litre tank would house 3 Corey's and a Betta nicely. Other tank mates include apple snails, and octolinius. So a Betta, an oto and a snail would be a good combination. I have always known mollies to be a bit nippy, not just to Bettas, but they bully plates too. Anything with bright colours is a mono with a Betta as he will see them as another male Betta and there will be carnage. Keep your 90 litre tank for the mollies and plates, and the 60 l for the Betta, heavily planted ( real or silk plants is best...no plastic plants as they will tear his fins). No enough hide holes will cause stress so plenty of cover. Include an almond leaf ( specialised , but easily bought from eBay) as it helps it's ability to fight germs.
Hope that helps x