I have 4 girls in 12 gallons, so 20gal for 6 should be fine.
Mine got along fine first 2 days, 3rd day they established hierarchy. Later, I added a 4th and she is now last. Only one Betta sustained fin damage(3rd girl) rest are fine.
Just make sure you check on them, feed them enough food and spread it out. Also, make sure there are hiding spots all over the tank, top/middle/bottom.
Lots of heavy planting would be advisable. This includes middle, back, sides, front, and even the top which can be accomplished with floating plants. Bettas are top-dwellers, after all. That's all I can really say. Good luck, make it pretty!