There are a variety of fish that will be compatible with mollies--especially if you stick with the cultivated types like lyre tails, which do not have the same preference for hard water as the closer-to-nature black mollies.
Lots of tetras that will work out--I'm partial to the blood fin and silver tips myself. If you have 4 mollies--one male and three females, you could safely add 5-6 of the small-medium sized tetras.
Some types of barbs are poor community fish. They chase and nip at tankmates unless there is a large enough school to keep them targetting each other, and in a 20, this means you'd really reduce the other fish you could have with them. Smaller ones like cherries would work, but again, you will limit the number of other fish you can have.
Kuhli loaches are very hardy fish, and I adore all of mine. But, be prepared to seldom see them. These fish are primarily nocturnal, and spend the day hiding out in caves and under the substrate. A group will be best, as they are very social, a 4-5 would do well in a 20.
I would avoid adding a redtail shark to a 20 gallon tank. These guys get large--6-7 inches easily--and can be very bad tempered, harrassing tank mates constantly. With the other bio-load, this would be too much, IMO.
A betta should be okay with these fish, other than the barbs. Those flowing fins are too tempting a target, and he will get nipped. You'll need to watch the mollies, as they can be nippish as well.
The best means of controlling algae is to limit the nutrients available for it to use. This means care when feeding, regular cleaning, and using plants to compete for the nutrients. Snails will eat algae, and I have them in all of my tanks--though I don't bother buying them. They come in on plants, and are mostly common pond snails.
Plecos (of most species) are poor choices for algae control. Not only do most get large, but they are prodigious producers of waste. And, the young are good cleaners, but the adults seldom eat more than the occassional bit of algae. Otocinclus are an exception--they are hard workers, prefer to be in groups, and stay small. However, they shouldn't be gotten just as a clean up crew, since they are hard to find in good shape.
