I'm so sorry about your girls! I would venture to say that you did not directly cause their deaths; they were stressed and sick to begin with. That first one especially, poor baby.
However, I am learning that sororities can be really tricky places, especially for new fish. Its' probably best to make sure all your girls are in tip top shape before you introduce them to the community, and then, when you're/they're ready, take everybody out, rearrange the tank, and then put them all in again at the same time. This erases the hierarchy. I think you have to do this every time you have an addition.
Also, even if there isn't any direct fighting, a new fish put into an established sorority will be stressed. They have to quickly defend themselves and attempt to find a place in the hierarchy. If the girl is sick to begin with, this could push her over the edge.
Again, I'm so sorry this happened to you. Hopefully next time will go more smoothly. Seriously, see how others do thier sorority introductions, maybe that will help.