Prefabs / meshes / instances / performance / size

Hellh there, I am making my game by using tiled things (repeating blocks of things that I did in my 3d software) to try to make the game smaller but I have a doubt:

1 - I exported some FBX with several objects inside, then I put the mesh on my Hierarchy tab. 2 - I manipulate my FBX sons freely, by moving, copying, pastiyng, scaling, rotating, etc. 3 - when I copy or delete one of the sons the prefab is broke.

The question I have is that if I break the prefab will I be still instanciating things (when I copy and paste one of my fbx sons) or every object will count as an unique object (making my game bigger in size??

may I make every FBX layer a separated prefab??

Thanks in advance