Basically what Sweeda said. You could also do a couple of African Dwarf frogs but in a 6.6 gallon you are quite limited.
A Chinese Algae Eater (further referred to as CAE for simplicity) is probably one of the worst possible additions to a Betta tank short of something that grows big enough to eat bettas. The CAE can easily and quickly grows to at LEAST 6 inches, they usually need something closer to 20+ gallons. They are highly aggressive and territorial and will beat up any fish in "their" territory. With slow moving fish that have large sides (like bettas, basically tall and skinny) they will actually eat the fish's slime coat and kill them.
90+% of other tropical fish either need at least 10 gallons or to be in small groups which need at least 10 gallons.
So as was mentioned above that leaves you with Shrimp, Frogs and Snails. All of which can be fun and rewarding additions.