I want to make some Stepping Stones with my Polished Rocks

I would put down about a 1/4 to 1/2 inch of fine sand at the base of the mold first. This will allow you to push the stones down in to the sand a little before adding the cement, which serves several purposes. First if the stones are not flat this will help hold the stones in place. Secondly, it will prevent the cement from going under the stones coveting them up when the step is popped out of the mold.

Also remember that if the cement is too wet it will be weaker. Only add enough water to make it slightly fluid.

You can also buy chopped plastic fibers, polyethylene (PET) if I think they were, or fiberglass to add to the cement for reinforcement. This will provide reinforcement throughout the cement. Any fibers sticking out can be easily burnt off with a propane torch.

And don't forget to tap of vibrate the mold to work air bubbles out immediately after pouring the cement mix.

You might also consider using tile mortar instead of cement. I find that the plasticizers and surfactants make it adhere to surfaces so much better than cement.

If you check places like Lowes and Home Depot they will often discount broken and torn bags quite a bit to get them sold. I stocked up on these bags to tile my floor saving me a ton of money.

I did something like this several years ago to make a border around some yard lights.  I could run the mower deck over the top of the "stepping stone" border  while going around the light fixture.  No more grass growing up around the lights that had to be trimmed by hand...

I cut a five gallon bucket into about 2 inch tall rings to use as a form.  I filled the ring about 3/4 with concrete, put in a layer of rocks about even with the top of the ring and covered the rocks with more concrete.  When the concrete set up about hard enough that I couldn't press my thumb into it, I took a stiff brush and scrubbed away concrete to expose the tops of the rocks.  When they were exposed to my liking I wiped the surface with a slightly damp sponge to clean off the rocks.  I don't remember if I could lift the rings off or had to cut them off, but it worked.  They were still solid when I moved out years later.  

It didn't bother me, but if your rocks are soft or glass the sand in the concrete might leave some scratches.  

I made something similar about 20 years ago. I bought rubber molds, glass and the the mix from a local craft supply. The mix was available in black or white - both are nice. The glass was random pieces of scrap glass left over from stain glass workshops. I sprayed the mold with a release agent and then assembled the glass into the bottom of the mold in the desired pattern. The mix was a powder (mixed with water and a hardening agent, into a thick slurry) and poured into the mold over the glass. It was allowed to set overnight and popped out of the mold the next day. About 2 weeks later and before putting the stepping stones outside into the elements, they were sealed with Thompsons. The thicker stones (ie. 2"+) lasted much longer than thinner ones. I set the stones onto (flat and tamped) crushed limestone and made sure they would not rock. I put down additional limestone between the stones. The only stones that have survived however were ones that I removed before winter and put into my shed.

Muriatic will work.

But I would avoid that hastle and danger by getting one of those big sponges tile setters use. As the cement cures, clean the stones with wet sponge. Any Haze left after that will again wipe off with a wet sponge after all is cured.

That's how I've done it. Seal it once you've gotten the haze off (sometimes a bit of scouring powder, scouring pad, or a tile setting sponge with a plastic scourer on one side will make that step easier). Never had problems with polished small stones being too slippery (and we have snow most winters), though I can see that being a hazard if the polished stones are larger.