I don't see why not, just understand that the unsanded has more shrinkage and thats why its really only for very thin grout lines. So that may give you trouble but I think mixing would help

My B-I-L does tile work and he suggested not to use sanded grout with glass that it might scratch the glass. Has anyone tried it?

I've not had any problem with glass and sanded grout.

I have heard it used mixed but never tried it. I'd just keep grout lines on the smaller size.
(I use sanded for all my glass projects w/o problems)