I had pebble base in my last shower and I loved it. However, I used rounded top (not cut top) pebbles and I really liked how it felt on my feet (very natural and comfortable) and there was never a worry about slipping. So I would suggest going for the non-cut pebble tile instead of the one you show. I only used the pebble on the floor and think that works best so other tiles would be needed to go elsewhere. I had 3 different types of tile, basically 2 accents (one being the pebble) and one subdued field tile. You should consider how this will tie in with backsplash/vanity tile too in that bathroom. (I would suggest using the same tile as in the niche in this case.)

One thing to consider with the pebble tiles is you will have more grout showing--no teeny grout lines--as it will go in-between the varied spaces between the pebbles.

I think the pieces for the master look nice together. Best of luck.

I love my pebble shower floor. Mine are flat like yours. I continued the pebbles for the niche adding a third element felt too busy to me. Have you considered that?

The pebbles are free flowing and somewhat amorphous, the cubes are heavily geometric.

To my eye, they clash, so I'll agree with skippack.

With that written, contrast can work as a design element. If that is what appeals to your design eye, then you've got it there.

I would put the pebbles in the niche too -- it will help unify the feel of the place. You already have different wall and floor tiles, so adding another element is best reserved for Serious Reasons, which you don't really have. Plus, having the pebbles up higher means you'll get to enjoy seeing them more!