If the potting mix already has a significant amount of vermiculite or some other moisture regulator / drainage component to it, you could get by with mixing some straight compost into the top portion to boost the nutritive organic component.

What is wrong with your native soil? Amend with compost and coffee grounds for a significant cost savings. I have done this both with sand and clay soils.

You can get away with 6-8" of depth (unless you want long carrots), Mel's mix is a well balanced combination and has been developed from much research, especially for shallow depths. Using some potting soil probably wouldn't hurt; I've done so without any problems.

Depending on where you live, native soil is usually too heavy and nutrient-poor for SFG, but is always improved by adding plenty of compost and vermiculite.
Good luck! Lynn