Being generous with adult lengths, you could add 2 panda cories to the 15 gallon. That's about it unless you remove more fish. You've got 2 gouramis in a small tank, that will probably cause problems and it is ~much~ more likely that one of the gouramis is responsible for your betta's death than the molly, and you stated before that you didn't actually see it happen, you just assumed it was the molly based on previous observations, then basically tortured the poor molly to death in retribution. Bettas and gouramis are closely related and will often try to kill each other, especially in such a small tank. You've got 2 kinds of solo cories and they need to school. You don't have room for a school of tetras.
I'd say, return one of the gouramis, one of the cories, and maybe the platy too if you want to make some room. Then buy like 3 more of whatever cory you keep (and i think you should keep the panda since they stay small) and then you could fit a small school of something like harlequin rasboras or glowlights.