No, they aren't replaceable. What you need is to have your nearest and dearest with you, not people to fulfill specific roles. If you bring in people who aren't your nearest and dearest, just because you think you need to fulfill those roles, it is a recipe for drama in the wedding party. So you can simply skip flower girl, ring bearer, page princess, and the extra bridesmaid.
This. All of this. Making new friends just so you can even out your sides (uneven sides is more than fine) is not the way to go and probably shouldn't be how you view your friends.
You don't need flower girls and ring bearers--couples go without them all the time. It's also totally fine to just have one ring bearer and no flower girl, three flower girls and no ring bearer, whatever combination is fine. Keep your wedding party as is. When you say "rearrange the party" do you mean just switch up how they walk down the aisle/who they walk down the aisle with? Because... yeah, you can do that.
If your "vision" is more important to you than real people's real feelings...then you should probably rethink your vision.
I'm with everyone else that even sides are not important and that people are not replaceable. Flower girls and ring bearers aren't even important (as far as any duties they perform) beyond wanting to include special children in your day. If there are no children that fill that role in your life, there's no reason to find some to include in your wedding.
All of that said, it sounds like you should just hire some models to complete your 6, 6, and 6. That will be the simplest way to reach your goal with the least amount of hurt feelings.