Chocolate gouramies are *extremely* fragile fish. Gouramies won't get along with bettas because bettas have a tendency to go for anything that looks similar to themselves - this includes long-finned colorful guppies and the closely-related gouramies.
Pencilfish wont work because they like to be kept in groups, so, like the cories, your tank doesn't have enough space.
A couple of male platies would be ok - I don't understand what you are worried about with the eyes? Only males though - so they don't mate or store sperm and give birth (females may become pregnant 4 months after being bought... even without a male because they can store sperm from a male at the LFS).
I still stand by my suggestion - otos - 2 of them, would make an excellent addition, are attractive and will eat algae too

Plus they like the lower levels so would balance your betta out nicely.
With mollies there are several factors to consider. For a start, they grow large. Larger than guppies or platies, and can reach 4 inches in extreme cases (usualy around 2.5-3 inches though). As such, I would only keep one. But keeping one may be a problem because a lone male can (occasionaly - it's rare but may be risky) sometimes become aggressive and nip (nothing serious but a betta wouldn't enjoy it) and you don't want a female because they can store sperm and give birth in your tank up to 4 months after being bought, plus they are larger than males.