How to share fields between commerce variation types?
It sounds like what you're wanting to do is create a group of product variations where you only have to specifically set the unique field value per variation. If that's the case, it isn't possible. We tried to implement some sort of field inheritance model where you could set default field values for each variation at the product display level, but it never developed.

The closest thing is the Bulk Product Creation module, which does let you create a whole set of related product variations at once. I don't believe it's been integrated into Inline Entity Form / Kickstart 2.x, though, so you may want to play with it first in a development environment before trying it in your live database.