Well, the more water volume in the tank the better, it allows for a bigger crash to occur in the system with less consequences. The platys will make babies, thats what they do, they can store male sperm for a few months as well, they will also eat most of their babies though.
The best you can do atm is return all the fish and cycle it, i know its boring but it will be much better for the fish in the end and you shouldnt lose any fish from doing this. Might be worth it to get a 30 gallon, you can have a wider variety of fish and schools, etc. etc.
You can leave the betta in the 10 gallon and watch the ammonia levels in the tank, throw some plants in there, have some ghost shrimp and maybe a rabbit snail once the tank is more established.
The cories, tetras and danios, however would be much happier in a 30 gallon if rehoming the fish is not an option, but i would look into an API master test kit so you can watch the ammonia, nitrite and nitrate levels in the tank. Ammonia and nitrite should always read 0 in a cycled tank, if nitrite spikes it is much worse because nitrites dont leave the fishes body as quickly. These two chemicals are literally poisons that will engulf your fish body and kill them slowly and painfully. Nitrates are the less toxic form of nitrite and should be around 0-20 ppm at all times, excessively high amounts can cause illnesses like fin rot, or other immune system disorders.
Honestly, fish keeping is an amazing hobby, just dont over think it, once your tank is cycled and you have a properly stocked tank all you have to do is make sure temp. is ok everyday and then feed your fish daily and you can even skip some days and it would be beneficial for your fish sometimes. For maintenance you just change 25-50% of the water every week with declorinated water and clean the filter media in old fish tank water ALWAYS, unless you have well water that does not contain chlorine. Atm i maintain 5 tanks, a 180, a 75, a 55, a 26, and a 20, you want to know which one is the easiest? The 180 lol I use a python siphon to suck out water for me and i fill it back up with the shower.