hi there, welcome to the forum and to fishkeeping.
i will answer your question about stocking but there's some info you need before you even consider what fish to get. Sometimes people don't know that they need to know something so they don't know what questions to ask!
the most important part to fishkeeping is keeping the water clean and healthy. a wise man once said 'we are not fish keepers, we are water keepers, look after your water nad the fish will look after themselves'. now it's not strictly speaking true, you need to feed them and stuff but the general idea is there. You need to take a little bit of time to understand the water chemistry issues prevalaent in fishkeeping and make sure the water is set up suitably before you add fish, if you do this then your whole fishkeeping experience will be much easier and better.
So a few bits of reading for you, these links are all in my signature if you make sure you read through 'step by step guide to setting up an aquarium' 'whats cycling' and 'fishless cycling' then post back with whatever specific questions you may have.
Right onto what fish to get, don't worry oveer getting the list finalised asap as you've some work to do before the tank is ready to safely add fish, it's just a couple of weeks so it gives you plenty of time to do your research and get a great stock list together. The most important thing is to do plenty of research into the species you want to keep and never impulse buy fish. Also remember when fish are in the shop they are often juveniles meaning they can be small but may end up very very big, so don't assume that fish are their adult size, invariably they will grow.
So to get a general idea of the amount of fish you can have the guideline is 1" of fish per US Gallon of water, now this is just a guideline, there's flexibility and exceptions to the rule but it works to get an idea. So you have a 20 gallon tank, you need to look up the adult size of each fish species you want, add them all together and they should be around 20. You add about two thirds of this after the cycle so you'd be looking to add around 10-15" of fish initially then building up the rest later.
also read the link in my sig 'guide to stocking lists' which tells you what to research and how to work out if fish will be compatible and suitable for your tank.
The only fish in your list that's completely unsuitable is the angelfish, it just gets too big for your tank. the small ones are just babies there's no 'dwarf' species of anything like that.
The fish that I would suggest you avoid are tiger barbs and dwarf gouramis. The tiger barbs get to around 2" each, they are very agressive and need to be kept in a big group to dissipate this and stop them from bnullying other fish. So you'd need 10 of them, 10x2" = 20" so a shoal of tigeters would be the whole stocking for your tank. I'm sure you want more than 1 species so I'd suggest they are out!
Dwarf Gourami's are a very weak disease prone species, while they're lovely in temperment and very pretty they are also very weak and a lot of people just struggle to keep them alive. I certainly wouldn't recommend them to a beginner with a brand new tank. You could consider the honey gourami as an alternative though which is a similar size and much hardier.
Onto the rest of the fish, you need to consider the shoal sizes required by the fish, the tetras and rasboras will need a group of 6. the guppies and swordtails need to be in multiples of 3 (2 females to every male) or an all male group 3+
the rams I would then suggest you just get 1 by itself as a feature fish for the tank.
So I would do something like this
initial stocking
6 x harlequin rasbora - 6"
4 x small corydora species - 6"
then after 3/4 months add
1 x ram - 3"
3 x guppies - 3"
the tank would then be lightly but reasonably stocked, after 6 months or so you could increase it by adding another small shoal of tetras or something along those lines.
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