The Shapton holder is nice, but not nearly necessary. The Stone holder Steve linked to is more than adequate.
Another great low grit stone is the Latte:http://www.chefknivestogo.com/la400grst.html
However, if you already have and like the GlassStones....I'd agree with Steve that the 320 is the obvious choice.
For a thinner knife than the Tojiro, you'll want something we label as a laser.
A good knife in that category would be the Kohetsu:http://www.chefknivestogo.com/rikoaosu24gy.html
It's carbon core and stainless clad.