You can indeed separate out a male guppy that has been injured. It is a good idea if you separate fish before treating them so that they have a good chance of recovering unmolested by the other fish. In an ideal world, each male livebearer will have at least two females to distract him in a small population mixed sex tank. The females can be stressed considerably by breeding attempts by the males in any smaller ratio until you get to the point where there are so many males and females that no individual female ever gets singled out for the male's attentions. A male alone in a recovery tank is fine, the ratio only applies when you have a mix of the sexes.