On the flip side, I personally don't like livebearers and never really had had any luck with them as a first fish. They do require a little extra salt thrown into their aquariums or else they'll get a nice fungal infection. Also, make sure you keep your tank temperatures at 76oF. Most livebearers don't seem to do well in high temps either.
I would still go with the 20 gallon tank though. 20 gallon longs are usually what's reccomended because the surface area for O2 exchange is larger, but it has a very limited viewing space. 20 gallon highs have a nice large viewing space, and that's what I have at home. To compensate for it being taller, I've added a TetraTec 30 air pump and a 12" airstone to help with bottom-to-top circulation. I turn the bubbles off at night though, they can sometimes get annoying when I'm trying to sleep.
Temperature wise, 76oF is good for all tropical community fish. Although most community fish will do just as well as low as 72oF. I don't even have a heater in my tank, and the fish are doing fine with temps between 70oF and 72oF. If you do decided to put a heater in your tank, be sure to place it within the stream of bubbles from your air stone to help with the circulation of the warmed water.
Water hardness and pH I usually tell people not to worry about unless they live in extream areas, are trying to do breeding, or want a very specific fish (African Cichlids, Rams, Discus etc). Most tropical community do just fine at a pH of around 7.6 to 7.8. Trying to use chemials such as pH Down to lower hard water pH is utterly useless because of the buffering capacity of your water. Check with your LFS, sometimes they'll let you test the pH of their tanks before you purcahse fish (bring your own kit just in case they don't have one), and buy fish from the tanks that closest match the pH in your tank. If your LFS is using the same water supply you are at home (county, city, whatever), most likely the fish at the shop are already acclimated to those water conditions, and you have nothing to worry about.
I personally love tetras. They seem to do very well in almost any water, and don't develop fungal infections as easily as some inbred livebearers do. They don't breed prolifically, and mix well with most type of fish (groumies, barbs, loaches, catfish, rainbows, rasboras etc). Livebearers, espeically the more brackish ones like mollies, don't mix well with tetras because of the extra salt they require.
Most people thing tetra's don't have enough color. That's hogwash. Decorate your tank with lots of natural greens, use a neutral gravel, and don't put in wild colors that will detract from the fish. Most tetras look stunning when seen in the proper light.
~~Colesea