Your female swordtail is probably already pregnant, so no worries on that account. And the males will continue to try to inseminate her any chance they get. It's not for mating that the 2:1 ratio is suggested. Males will harrass females intently in their efforts to mate, and so having multiple females will reduce the harrassment each INDIVIDUAL female receives. Also, males will often compete with each other for the right to mate with females, and this can get dangerous, resulting in injury or death when it gets severe. Having more females than males means plenty of females to go around, and, while the males may still fight, it is much less likely that they will do so.