Hey romaric, just going to give you some info on the list:
Black Skirt Tetra: Grows to 2-2.5", enjoys schools of 6+, nips. Do not place with angelfish, bettas, guppies, or other long finned fish.
Callistus Tetra: Haven't heard of this one, I'd skip it if you can't get a comfirmation on what it is, but someone here probably knows what it is.
RTS: Not recommended. Grows to 4.5-5" min, so it's already quite big for the tank. They also get to be some of the most agressive fish out there, and would murder any bottom-dwellers.
Convicts: These guys grow to 6", and are an extremely agressive cichlids, barely able to be crammed into a 30g. They'd rip apart almost any community fish I know of.
Mono: Another bad one. These get huge, and salt water (high end brackish is okay), need schools, and, really, should be kept in a very large tank (over 300g min).
Guppies: Perfect starting fish, the best out there. Tough as a nail and pretty, too. Recommended you get males only, they're prolific breeders and also known as the "Millions Fish". Females can bear young many times after seeing a male, even if he's gone. Females are larger, and have shorter fins and a lot less colour than males. Very cheap fish, sometimes 2$ or less. High end "show" guppies can get pricey, 20-25$ sometimes. Guppies reach 1.5" for males, 1" or so without the tail, and females 1.5-2".
Cardinals: Another great biginner, these fish should be kept in schools of 6-8 minimum, and usually swim in a tight swarm. Very colourful and pretty hardy. Get somewhere around 1-1.5", and are usually cheap.
Harlies: These rasboras I do not know a great deal about. I know they are pretty hardy, get around 1.5", and are common and cheap, and I guess that's all I really need to say, lol.
Some suggestions:
Lemon/Neon/Blk neon/Glowlight tetras: All of these very around the length of 1.5", and school like the Cardinals. Usually cheap.
Cories: Some of my favorite fish, peppered = 2-2.5", panda = < 2", bronze = 2.5-3", albino = 2-3" (depending on weather they're peppered or bronze). Always cheap (got mine at a sale in New Minas once for 3 or 4 for 12$, good deal) and can only hurt pellets and fish food.
Otos: Don't exceed 3 in a 20g, add when the tank is established. Good algae eaters that get 1.5-2" and look pretty cute too. Cheap.