I mean you don't need to study up on each foundation, that's what training is for, but at least familiarize yourself with the letter and numbering system, and maybe get to know 2 of the foundations (studio fix fluid and maybe studio sculpt).
Tiffany aka prettysecrets DID get hired and she didn't use foundation, however I don't think most counters or stores would be that lenient. The whole point of the demo is to show that you know how to do makeup, so even if your model has amazing skin, I'd still put on a sheer foundation and set with powder and do the whole 9 because they need to see that you know what you're doing and that you know how to apply.
Also, you mentioned you have great skin as well. You're still going to be expected to wear foundation and powder to work. They have guidelines for how many products you need to wear (lipliner, lipstick & lipgloss, at least 3 shadows, etc)
Maybe you can go to another mac counter that you don't typically go to and ask to be matched up then watch them in the hand held mirror (or wall mirror if you're at a store) and see how they do it. and force yourself to wear it on yourself at home or whatever so you can practice, you'll get it, no worries