Yes you can leave then in the 10 gallon tank for the time being and they should be fine. Were you able to save the filter from the larger tank so the tank is cycled? If you could save the filter media anyway it would help to keep from having the whole cycle lost.
I hope this works out and all of the fish are fine.
You mentioned that Shadow had finrot and you were using "drops". Can you tell me what type of drops and how many you are using? I used to use meds on the bettas who had fin problems but none of them were drops. The only things you cannot use on him are Melafixk, Pimafix and Bettafix as those are toxic to bettas Labyrinth organs eventually.
Rose