Hey Jimmy! I have two girls and they do not share well. lol
I have heard that boys-girls get laong better than 2 girls though.
For us, we do seperate crates and beds to ensure safety and my older girl can get some peace when she needs it. I think they should be supervised when together at first. That will allow you to see how they will interact.
Have you thought about how you will introduce your new guy to Maggie? Doing this properly can ensure Maggie does not feel threatened by the new kid. HAs Maggie met her new brother yet?