Guppy, I have black skirt tetras. Unless you find some that are alot more peaceful than mine, they'll aggravate the platy pretty much constantly. They may be fine, though, I can only speak about my experience with the little buggers.
For a ten, why don't you get yourself some dwarf platies, or maybe a small tetra like glowlights, or maybe try again with some male guppies. A school of small fish can be very entertaining in a ten gallon aquarium. Then, when they've done well for a couple or three months, and you are able to get your 20 up and running, you can then add in a second school of a compatible type of fish, or one larger centerpiece-type fish.
It would seem to me that the goal right now would be to use your ten gallon to get a small community established, hopefully disease-free, so that when you get ready to, you can transfer your fish and the filter over to the bigger tank without too many problems.
Keep it simple, learn to keep your water properly (you have a good start, I know you've had some disappointing setbacks, but you'll be fine if you go back to basics and take it slow), and enjoy your fish.
Building slowly is a really good idea in this hobby, IMHO. It takes a long while for an aquarium to become stable. After you introduce any new fish, there's an adjustment period. We're often tempted to proceed too quickly, and that can make it harder on us. That works out better for some than for others. I was introducing new fish every couple of weeks into my 55, and even after quarantining them and with regular water testing, it still wasn't going well... ick outbreaks and such. Now, however, I'm waiting longer in between, maybe a month or more. For my tank, that works better. Each situation is different. You'll have to find what works best for you, but I do think it will involve loads of patience. I admire that you hang in there like you have. Patience and determination will win out in the end.