The split-face(d) travertine mosaic tile border we used on our tub surround walls has practically non-existent grout lines also. Our tile guys packed as much grout as they could in the voids and worked very carefully; methodically and slowly to make sure any spaces were filled/covered. Also, the border is high enough, so though it will certainly receive some water spray during DH's daily showers (I shower in the other bathroom), it is rarely, if ever drenched with water.

Oh, we made sure to seal the surface really well, too (prior to install, prior to grouting, and then after grouting).

The plan is to use this tile for all the walls and the floor as well. I can do something else instead on the shower floor but what are the thoughts on using this tile on the wall all the way down to the floor. I plan to use the schulter-kerdi system in the shower if that makes any difference.

I have calacatta marble mosaic on my shower floor and it also has very very tight joints.

Since natural stone is porous (and so is grout) water is going to get underneath it any way you look at it. With those teeny tight grout lines I do not think there is any way you could really totally seal it and make it 100% waterproof on the surface. The waterproofing below (your Kerdi) is what will deflect the water to the drain. My installer (Bill V.) recommended NOT sealing the mosaic floor. The stone and grout get darker when we shower, but dry out afterwards. I think any time you have porous natural stone that gets very wet in a shower, this is going to happen. But I could be wrong :)

could you post a picture of your mosaic tile for me to compare to mine? I would post a pic of mine but dont know how...

It's the marble, in the niche and shower floor... In the bottom pic you can just see how some of the marble pieces are actually touching each other...