I have harlies, I'd agree that 6 is pushing it in a 10, but if you're filtered well and are vigilant about changes (live plants too even better) it could work out.
Behaviorally they will appreciate some "blank" swimming space along with some broad leaved plants somewhere in the mid to upper level. When they spawn they zoom all in and out of the vegetation, chasing each other; and the females will upend themselves and rub their bellies on the undersides of the leaves. :) I wouldn't worry much about ending up with babies...eggs get eaten fairly quickly. :( I believe one of my links in my sig shows a video of this.
As far as the betta, I will say what I always do; depends on his temperament. Harlies can get quite brightly colored orange when they are happy, and while sometimes mine are pretty "chill" sometimes they are the opposite; very zippy, when they are courting and when they are eating. In the mornings when I feed mine I hear the jaws music in the background, I swear. Very fast little voracious buggers. It may or may not stress your betta out. Just something to keep in mind.
Last edited by dragonflie; 08-26-2011 at 01:27 PM.