neon tetras are schooling fish and need a school of at least 5-6 in order to feel comfortable and happy, so i would highly recommend getting more neons, or rehoming them if you cant. I dont know as much about mollys and swordtails but ive heard people say that they can do fine on your own. What are the dimensions of your tank? Need to know that before figuring out if a pleco would be suitable for your tank size.
Plecos- even the small ones- have a massive bioload. If you were to add one you need to take that into account. I am thinking of adding a BN plec to my 40g, according to aqadvisor that one fis would account for 23% of the tanks capacity. So in your 20g it would probably be closer to 40%, you would need good filtrate and light stocking.
I think 29 gallons is the minimum bristlenose tank size, mine is currently growing out in a 23, moving to a 32 after that. although I believe they are good algae eaters. Mine has cleared up at least four different algae types.