I believe White cloud minnows like colder temperatures than you'd want to put a betta in.
If you're talking 6.6 gallons, you could put in some snails or shrimp, but that's really too small to have any other fish besides a betta, since most small fish you'd need to put in at least 5 to make them comfy. If it's 6.6 litres, I wouldn't even put in shrimp or snails.
By the way, if you're doing water changes and planting with live plants, especially some fast growing ones, you shouldn't have much of a problem with algae.