Before considering a sorority, definitely up the number of neons to 6. :)
If you do want a sorority, bear in mind that it isn't a natural way for them to live. Girls in the wild don't live together, although they will commonly experience each other on their travels between males. It won't make her feel more comfortable. However, she may still be quite happy with the situation. My girls seem happier in their groups, but they have been living in groups since they were born and are pretty wierd little gals anyway. For most girls, such as yours, it isn't the norm.
If you do want to start a sorority, make sure your tank is incredibly densely planted. Because your girls wouldn't be sisters and probably wouldn't be used to being together, you really need dense decoration, all the way to the surface. Before adding your other girls, cup all the females and re-arrange the tank, then add them all back in together. Keep a close eye on them and have spare tanks on hand, just in case. Make sure you have at least 5 total, 6 being better.'
In your tank, it might not be a good idea, simply because your pleco has a pretty large bioload. Adding 5 or 6 girls to the mix may not be a good idea - at the very least, it will mean more work for you. It's also quite possible that girls won't get on with your guppies. Giving you this advice, however, does make me a dreadful hypocrite as my own tank is terribly overstocked, so I'll understand if people bulge their eyes at me reading this.
Basically, if you do want to start a sorority, I'd get a bigger tank. You could do it in your current tank, but bigger is always better. :)