mix and match model parts

Can I put a turret from one model onto the socket of another model ? or are units one whole model including their turret ?

You can import them into a modelling program and select verticies and then move those verticies on another socket.

So yes you just need to know how to manipulate meshes.

Is there a program you would recommend for someone who barely knows what a polygon is ?

Blender is a good one, and it's free.

And you need to install the blender importer/exporter for papa files.