Well, I'll let the experts weigh in, but drainage is really important for semps, and putting rocks in the bottom really doesn't do much to help the drainage. Your mix might work if you can restrain yourself from overwatering and leaving the soil too wet. That soil would be fine if you mixed in some perlite, pumice or very coarse sand. Fine play sand doesn't work.
I love to water, and tend to over do it, so I've taken to using Cactus mix with the extras mixed in for almost all of my plants. I do it for the semps, cacti, but also for my tropicals and other flowers. It saves me from rotting and drowning plants!
