u/Avallon50

I'm new to the hobby- so I'm not sure if this is a totally obvious question. I would like to include a lieutenant with combi weapon in my first tabletop army. It is fairly expensive and I'm not a fan of the original model from the Leviathan box. I would like to know the following:

1. What are the rules for kitbashing? Can I take any extra sword brrother models I have and modify it to be the lieutenant so long as I give it a combi weapon?

2. Where can I find a combi weapon? I just purchased the black templar upgrade kit, and I own the Combat Patrol 2025 box.

Thanks!