I bought one of those 20g starter kits to begin my entrance into the hobby. I now wish I'd splurged and got the 55g kit. The kits are fine for beginners--you are right to avoid lots of plants as those hoods have about 15w.
As for stocking, it is important to remember that most rules are too simplistic. For instance three fish in a 10g could work--3, 4, even 6 or 8 neon tetras or white clouds would work fine in a 10g. But 3 oscars or pacus would not physically fit in the tank. If the pleco is a common pleco it will not fit for long in the 10g, even alone, the most commonly sold variety of pleco can get way bigger then a foot long. Depending on what kind of tetras you could put a number more in the 10g and return the pleco to the local fish store.
There are a variety of combos and options for a 20g. I have two 20gs. One is a tiger barb species tank--they are nippy and do best in big schools, species only tanks, or in a big tank. The other is being used as a qt tank, but I had a dwarf gourami, five bronze corys, and I was going to get a small school of rasboras.
Take a look at what other people have in their 20g, to see if anything appeals to you. Remember that it isn't just size of the fish, but compatibility, water and temp needs, and so on. Think about what part of the tank they use as well.