Sure, you can mix just about anything in hypertufa. I've used chicken grit, marbles, sawdust, gravel, etc. The reason this project is so smooth compared to most hypertufa is that I used a concrete tube for the bottom also. The PVC pipe was inside the concrete tube and the hypertufa is about 3" thick around the PVC. I really packed the hypertufa in with a wood 2x2 between the tube and PVC to get it nice and solid. The top was made in a large tub and I also packed that down really hard.
I use peat moss that hasn't been sifted. I end up with larger chunks in the tufa and I like the look when it wears away. A lot of people sift the peat moss first, I only pick out sticks and break up really big chunks.
