Sand is definitely the way to go for cories - nicer for their barbels and it's so cute to watch them play in it. Just be aware that with sand you need to poke/stir it on a regular basis to prevent gas pockets forming.
Huzzah on the ten gallons, they are fine for cories and a betta. :) What sort of cories do you have? If pygmies, get a school of 6-8 per tank. If pandas, 4-6, and if the larger species (bronze, peppered etc), 4 is the number. Remember that they need schools of their own species of cory. :)
To speed up the cycle in your new filters, transfer the old filter media across, since they will already be colonised with some good bacteria. :)
You seem like a really good fish parent - ready to keep on upgrading for your fish! :) It's lovely to see.