I'm following this, because I just set up my new 20 long this weekend! It was an upgrade to a 10 gallon, and currently houses 1 betta, 7 dwarf cories, 3 adult Japanese trapdoors and 6 babies (yep, they bred in the 10 gallon). I'll be adding 8 more cories today to fill out a shoal of 15. They LOVE the new space, and it's so fun to watch them play!
All the plants from the 10 gallon got thrown in the 20 long, but it still looks a little sparse. I'll try to post a pic tonight or tomorrow - maybe the aquascapers on here can give me some inspiration.
Honestly, 20 long is one of my favorite tank dimensions. That, 75 gallons, and 125 gallons. All great dimensions for communities and plants, IMO.