Okay so I have kept bettas for like 15 years, and have 22 plus a batch of fry right now. 10 gallons is perfectly fine for a sorority, minimum is actually 4 not 5.
This being said, to have a successful sorority you MUST have tons of cover, whether it be live or fake plants, driftwood or caves, you have to have a ton of things in the tank to break up the line of sight.
As for Endlers, I wouldn't recommend them with bettas in something that small. I have tiger Endlers in my 26 gallon sorority tank and they do fine, but that's in a 26. I would never try them with a male because they are so brightly colored the male could easily confuse them for another small betta.