Mike,
as most acrylics dry to a nice dull/matte finish you should have no issues getting the pastels to adhere to your figures. I use them on armor models air-brushed with Testors acrylics with great results. I'm sure the finish on them is far smoother than your figures with hand brushed paint. I always make sure to do this as the last step, however. If you decide to spray your figure with any type of fixative or clear coat it will effect the pastels, it will diminish them quite a bit. What I would do is decide you are done with pretty much everything, spray your final dullcoat, then come in with the pastels as a finishing touch. Then make every effort not to touch them if at all possible! Aftermarket "weathering" powders have what they claim is an "adhesive" in them so they tend to have a bit more "staying" power than standard pastels. But both will be diminished by handeling and such as time goes on. The good news is, if you do them absolutely last, there is nothing stoping you from re-applying them in the future! If a dusting session gets out of hand....just give 'em another shot of pastels before show day! HTH,
Jay H.
OKC