Dave,
You're spot-on as far as a flat coat goes before applying pastels. However, if you're going to top-coat the pastels with more flat, you will see a very dramatic change in the appearance of you pastel work. You will notice that the pastel effect is seriously reduced and end up having to reapply the pastels several times to get a similar effect to the original pastels alone.
"How do you keep the pastels from rubbing off," you ask? Well, you don't. The final pastel application will rub off, but that's also an incentive to admire the model without handling it. A periodic touch up with some additional pastels is better (IMHO) than hiding all of your pastel work behind a coat of clear laquer.
Good luck on your build,
Foster