Tiger barbs arn't aggresive they just love picking at long fins. Most of the time they do it is when they're in too small of a school and are bored. I wouldn't put tiger barbs in anything smaller than a 30g long. You could put 3 maybe 4 female betta in a 10g if you kept up on water changes. Also the shape of a bettas tail has nothing to do with agression. Its like saying an albino BP is more agressive than a normal.