Scaling doesn't work well with assemblies, if you want to scale down/up an assembly then do each pc individually, however, in doing so you will need to create a new assembly and mate all the parts.
When you scale a part it scales from the plane in the first sketch, so if that plane is away from the point of origin, it will pull towards the plane, therefore when you scale and assembly with multiple parts it will look like an exploded view when done.....
NO but you can save assembly as part and then scale the multibody part.