Are you talking about that partial wall made out of block, near the street lamp? If yes, I would use the same tile as the other wall. If you use something different it is going to look like you bought remnants and didn't have enough for both walls.

Are those "slits" in the 2 walls going to be waterfalls? That is going to be really loud with all the fence/walls and hardscape around the pool.

Yes, those slits are waterfalls, and there is another waterfall at end of the pool.

Yes, I am referring to the block wall near the lamp. Could I use a different shape of tile? The tile on the larger wall along the pool is Riato Grand 2'x4', could I use Riato Grand Hexagon Mosaic on the block wall? See the pics below. Just want to make the two walls look a bit different.

Thank you!

I would use the exact same tile and size. You will have enough going on and I don't think you want to highlight the waterfall walls as being different. I also wouldn't want more grout to have to clean outside!