What molly species do you have now? The common black molly and its variants is a largish fish, males attaining 3 inches easily and females 5-6 inches. The reason they rarely do is because they die prematurely due to poor water conditions. The molly is not a "hardy" fish.
And if you intend those other fish, and/or have the Common Molly
, I would go beyond a 20g and consider a 3-foot (i.e., around 30+ gallons).
This equates to your question of 20g long or high. The longer the tank, the more space for the fish (the fish's length relates to this, plus its temperament/territorial disposition, etc), and the larger the surface area. All else being equal, a longer tank can safely house slightly more fish than a tall, even with the same volume. So the longer the tank the better.
Live plants are good; in the harder water Corkscrew Vallisneria
is ideal. It is in our profiles, click the shaded name. Floating plants like Water Sprite
, Brazilian Pennywort
, Dwarf Watter Lettuce.
Substrate: sand or fine gravel, darker the better. What are your water parameters, GH and pH? If this needs to be raised (molly must have medium hard or harder water with a higher basic pH) the substrate is the best way to do it, as you can get a calcareous sand/gravel. If corys are intended, sand is better.
I would not include a Bristlenose Pleco
, this will compete with the mollies for algae. Most other pleco get too large for tanks under 4-feet.
I would not suggest two filters; one that is adequate for the tank will suffice, and the plants help too.