Well everyone agrees get the largest tank, no bigger than a 55gal to start, that you can. Keep in mind it is floor space not hieght that matters.
All the fish you listed can go together. I would strongly suggest you check out Celestrial Danios before making a final decision. They put the rest of the danios to shame. Avoid Giant Danios as they can get nippy. A 30L-55gal can hold a good size danio shoal of 10-12 fish. Their bioload, poop factor, is low so no worries.
The livebearers, guppys, mollys and swords can all hybridize. I'd go with guppys and swords to cut down on the likelyhood. You could also go with all males of each type. Can I suggest getting some Endlers Livebearers and swords or mollys? The Endlers likely can't hybridize with the swords or mollys nearly as well as guppys. I can also tell you that a group of 6-10 Endler males would look awsome with some nice contrasting swords. Just a side note, swords and mollys can get up to 4 (+) inches and male mollys can be sort of aggressive. The swords, mollys and platys for that matter all have moderate bioloads-Mollys are the worst due to the veggie content of their food.
As for tetras, Neons or Cardinals look very nice, but the size of your other fish would lend itself to Cardinals over Neons for durability sake. Either way a shoal of 8-12 or larger is not pushing the limit and the bioload is low. I would also suggest you check out Penguin Tetras and Harlaquin Rasboras. Two shoals of 6-8 each looks very nice. Lemon Tetras, Penguins, Glowlights, Head and Tail Light and Black Neons are others I would look at. Stay away from Serpeas, Black, White and Skirted Tetras as they are fin nippers.
For your bottom dwellers, nothing beats cory cats. Do yourself a big favor, don't go with regular Aenues Corys, go for Pandas, Julis, Paleatus or something different. In a 55gal, two shoals numbering 6 of different corys would look nice and is doable. If you plan on adding plants, like java moss, then you could also get some Red Cherry or Blue shrimp for the bottom along with the corys. The other fish will eat most of the baby shrimp, but a few will survive to keep the group going. Depending on the aquascaping, stuff on the bottom, you could get a pair of CPO Dwarf Crayfish. They are too small to really bother the corys once full grown.
Last but not least, there is no reason not to try a betta. I have bettas in with every single fish mentioned except the danios and that is because I don't have any danios now. You could also go with pigmy gouramis such as Licorice or Croking Gouramis. They are very colorful and won't cause problems. I'd avoid the regular and dwarf gouramis due to the potential for aggression. If you are compelled to get a full size gourami get one male Pearl Gourami.
Keep posting up and good luck. If you have questions post up or you can P.M. me.