Honestly, I have a 5 gallon divided with two boys in it, and everything is fine. No undue stress, no outrageous amounts of flaring. They're happy, and each has decor to hide in for a break if they need it. I also know of at least a few members who have 10 gallons divided 3 ways.
That being said, I find that I enjoy the look of community tanks more than divided tanks. I wouldn't have divided my 5 gallon except that I got a surprise fish and had to figure some stuff out space-wise. So you could definitely divide the ten, but if you're looking for opinions, get one male and start a small community tank. You could do some cories, the betta, maybe some guppies?
Back after a hiatus.
Pongo - HMPK Koi Male
Onyx - DSH Cat