Potting soil will work if amended to increase drainage. The addition of crushed granite, pumice or a product called "dry stall" (or whatever equivalent the feed stores carry) will work. The people in the c&s forum love this formula called "gritty mix" but keep in mind many are growing in containers in colder climates. Just increase drainage and monitor your watering until sedums are established, then you can slack off. Sedums are really fairly tolerant plants IMO.

-Ming