I would NOT use HT for stepping stones. I've made stepping stones with my own concrete mix, and they held up several years, but are beginning to break down.

I've starting using simply plain ole Quickrite for my projects now, and I am VERY happy with it.

If you are concerned about a "rocky" surface with Quickrite, you might want to use their sandmix on the top surface- though if you are "anal" about tamping the standard Quickrite mix, you should be fine without it.

I don't know whether your instructions include placing support in the stones(?); regardless, I would highly recommend placing 2-3 pieces of rebar in the middle of each that you cast.

And I don't know how you want to do your shapes..... I made mine via a sandcasting approach, differing the shape of each stone, and "fossilizing" leaves, snakes, etc in the stones.

Good luck, and let me know it you have any question about my feedback..... but back to using HT, it simply will not be strong enough for stepping stones.

I made stepping stones 10 years ago from HT with sand, peatmoss and cement, 2,5-3 inch thick , we walk on it frequently and it is strong.

Big stones, appr. 30"x20" , just chicken wire for support. I made Japanese style arching bridges, 4" thick, appr. 4'x 2', again just with chicken wire , but under the arch there is a big stone in the rock-river to support it.