What you're looking to start is called a Betta Sorority, which I'm afraid is a little more complicated then just adding a couple of new girls. It can actually be very tricky, requires a lot of research and planning, and is not always advised for the beginner keeper.
Its important to remember, first of all, that the species of bettas you normally find in stores, which is what I assume you're working with, are a very aggressive and territorial species. While the females can be a tad calmer, they can also be just as, if not more, aggressive then any male and while they can sometimes tolerate each other in large groups in the right conditions, sometimes even in the perfect setup things can go wrong in the blink of an eye so its important to be prepared and have a backup plan.
What you'll want to do first is a LOT of research(here is our wonderful sticky on the subject if you haven't seen it already: http://www.bettafish.com/showthread.php?t=29402
and there are TONS of other great threads about them all over the forum)and preparation. You will need a good amount of live and/or silk plants and a few caves to break up the line of site and give the girls a place to get away from the others, and you will need to have a group of about 5-6 in order to spread out the aggression.
This could however pose a bit of a problem stocking-wise though with your BN pleco. Since Plecos tend to produce a lot of waste(and actually get a little large for a 10 gallon anyway), if you really would like to start a Sorority, you may want to look into finding him a new home or upgrading for him. A proper Sorority and your BN would be a little too much bio-load for a 10 gallon.
Also, assuming your 10 gallon is filtered, and therefore cycled or in the process of cycling.....a 100% water change isn't necessary, nor would I recommend it. That'll just crash your cycle, so unless you want to do that and start over, if you decide to do a Sorority you can just do a large 50%-75% change when you do your redecorating(which you should do just before adding all of your girls at the same time).
Best of luck!