Lots of good replies here.
*I think the best one is the School Glue. The glue will hold the fabrics where you want them until they are sewn. It will help keep your seams nesting also. I'm sure the minky is hard to keep nesting as it goes through the serger.
*If you make an off-set seam, you will still want that to be mostly in the same place on each row. Pinning a lot will help, but you have to take the pin out before you put it through the serger.
*If you put sashing on, you will still want the seams to line up on each side of the sashing.
* From what you said about cutting your blocks 6 inches and adding 1/2 inch seams, your blocks should come out to be 6 inch finished. Do they? That doesn't really matter if you are consistent with the seams in the whole quilt.
So I guess I would glue the seams first, sew on a regular sewing machine, then serge those seams before they are added to another seam. If you have 1/2 inch in the seams, that's plenty to put through the serger.
You will learn a lot here from a lot of different people. Then do what you want to do. It's your quilt.