Tile spacing both above and below 1/8", what type of grout should I use?
I would use sanded grout.

You will have to work harder to force the grout into the narrower gaps, but it will fill.  It will self strain the sand particles out.  Try wiping left-right and diagonally with the grout float pressed very firmly to the tile

The admix is a good thing.  It makes for better troweling properties, which means it will level better.