So I have a small sorority of bettas in a 15 gallon tank; my smallest girl is being nipped relentlessly (Freddie). Because I have other fish in the tank, is it potentially safe to take Freddie out for a week or so to grow back her fins, or should I not even risk having two female bettas together?
You should not even risk having three females together. The minimum for a sorority is five girls, and they should all be added at the same time so that the aggression will be kept to a minimum. Also, your tank should be extremely heavily planted (either live or silk plants, both are fine) and have tons of hides and plants to break the line of sight.
I'm not sure how your stocking levels are right now, since I have no experience with plecos and limited experience with danios, but you need to add at least another two girls. In a fifteen gallon, though, you could easily have about ten girls. You will notice a drop in aggression, the more females you have.