The environment up on a roof is unlike any other on earth. There are many different factors that will determine the type of soil to use and the assortment of plants that can be planted and expected to thrive. The biggest of these is the building structure itself. How much weight can the roof hold up? The more weight you want to put on your roof the stronger the building and roof need to be. The stronger the structure the deeper the soil and a greater variety of plants can be added to your planting pallet. Water, sun and wind are also considerations. Will you be installing an irrigation system? If you do you can use plants that perhaps need a bit more water. Some plants need some shade to do really well and generally there's a short supply of that on a roof (depending on adjacent buildings and really large trees of course). Below is a list of plants that we have used successfully on roofs. I've only included plants that are drought resistant and are able to handle full sun.
The toughest of the tough! These guys can handle 2 to 3 inches of green roof soil.
A little more love for these ones but still super hardy in 4 to 6 inches of green roof soil.
Not exactly soft and fluffy but they do like deep soil at over 6 inches.