bettas will ALWAYS be aggressive to each other. the videos on youtube, don't follow them. often, it's chance. they take the divider out, video tape the first few moments, then the video ends. often, it takes bettas anywhere from a few minutes, to days, but they'll always attack each other.
there's no 'least aggressive' type of betta. it depends on the fish. Cup, when he was still with us, was VERY aggressive. he'd flare at everything, including sounds! but, Ichi, my delta, never flares. i gotta work hard to get him to flare at anything, and he actually lived beside my blind betta, within sight of him 24/7, and had no issues.
some bettas, can live with ADF's. some, won't. it depends on your fish.