How much should i mask when working with 2 colors?

Probably the only problem will be black then white.

Generally speaking, it's better to apply lighter colours first, then the darker. (Dark covers light better than vice-versa).

Masking over metallic can be a little problematic as metallic finishes are more easily marred and they may be more easily lifted by masking tape than solid colours, but sometimes it depends on the paint.

Take the above advice, lighter to darker, etc. Another good idea, looking at your subject choice is to build and paint in sub assemblies as much as possible. Thgen assemble the seperately painted sub assemblies. It looks like that will reduce your need for masking, if that thing builds up like Gundams or similar types of kits.

Leaving unmasked areas can be a problem. If you are very careful, you will get away with it most of time.  But you leave yourself open for a slipup.  One easy thing I do when I have to do just a small spot and the rest of model is unmasked is to wrap some kitchen wrap plastic around it. It will cling well enough that it will prevent paint from hitting unmasked surfaces.  Maybe tape just a small area around where you have to spray.