i think you should transfer everything from your 10 gallon into your 20. for substrate you could do some Schultz aquatic soil from homedepot as a bottom layer and your gravel up top. i did this in my 20 when i upgraded from a 10 and i couldnt be happier
also i have some plants i could send your way. moneywort, some leftover anubias rhizomes, hornwort