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My favs that work for a 20 gal would be...german blue rams or bolivian rams, gardneri clown killifish, scarlet badis, glowlight tetras, julii corydoras, glo fish/zebra danios, striped kuhli loaches, betta splendens, neon blue dwarf gourami.
I have 7 in my 20, they are great. And so far they seem much hardier than the neon tetras. Remember to try to fill out the different levels of the tank. Tetras for instance hang out at the bottom mostly while a gourami is at the top alot..try to keep it even so your entire tank is being utilized and no one is crowded.
In my experience, guppies and platties are pretty hardy, and are good options to fill out the top and middle. Guppies are one of the most colorful fish ever. I have a group of 10 and each one looks totally different and they are really active.
They aren't more demanding than any other fish IMO. I don't like the term hardy, as that indicates they will withstand abuse (ie, poor water parameters). But, they're not a sensitive fish like, lets say, discus or rams.
My tank is a 40 breeder planted. You can see the most recent updates on it in the link in my signature (you'll have to go to the next to the last page I think)
Mollies aren't well suited to 20 gallon tanks. They just get too large and produce too much waste. If you did really want one, I would maybe get one, not 8. 8 mollies alone would overstock this tank without anything else added.
Fancy male guppies come in all different colors and without any females you don't have to worry about your tank getting over run with fry.
To sex livebearers (guppies, platties, mollies) look at the anal fin. On females, there is an actual triangular fin. On males there is a more elongated "fin" that they hold in close to their bodies. You can look up pics if you want. I would recommend you get familiar with sexing them before you go out to buy fish so you can choose all one sex of any livebearer.