I have never had a Betta that didn't thrive in a community tank except three that were too aggressive for tank mates of any sort.
You can have seven Pygmy Cories in a six gallon. They are quite tiny and, at least in my tanks, left completely alone by the Betta. I have three Betta-based community tanks and have also had success with small Rasbora (Sundadanio axelrodi, Chili or Sparrow) in smaller tanks.
I would stay away from some Tetras as they can be very nippy. I thought I had a tail biter until I saw a Neon remove a chunk. While I love them I'm in the process of finding them a new home. The Ember Tetras don't seem to be as nippy as the Neons.
If you don't mind feeding live food, a wonderful, colorful little fish (and my favorite) is the Dario Dario aka Scarlet Badis. They are less than an inch in size but do require something like live Micro Worms to do best. They love moss. Do not feed frozen bloodworms as a staple diet.
An interesting alternative would be Dwarf Crayfish (Cambarellus patzcuarensis sp orange or Cambarellus texanus). You could have three or four as they are peaceful but can be territorial with each other. Do not get any other variety of Cray. I do not recommend shrimp of any sort.
Three African Dwarf Frogs would be work as their bioload is no more than a large Tetra. Target feeding is required if you choose frozen bloodworms. I feed mine HBH Frog and Tadpole Bites and the occasional frozen bloodworm (using tongs so piggy Random the Betta doesn't steal). However, they absolutely cannot have freeze dried foods as freeze dried will cause fatal blockages.
If you want colorful, you could go with five male Endlers or three male Fancy Guppies. It's pretty much a myth that Betta can't live with colorful, long-finned fish or see them as other Betta.
With the addition of any tank mates, keep an eye on water parameters as you may have to up the number or percentage of changes. And, as I'm sure you've seen it preached
have a back up plan.
Have fun choosing!!!