Go with a single stage automotive paint. That will go over Por 15..... just go to a auto paint dealer and ask.
You can put any additive you want into paint regardless of flake, pearl, flex.......
Why do you want to try for a high(er) end paint job and go cheap on the paint? The work and prep are all the same, you're just cheating yourself by using a paint that will perform at the low end of the scale when compared what's available. I'm not saying Rustolium is terrible paint, but with all work involved it may pay to go with a better product.
The reason I think its odd is because you asked about the least expensive base coat with a supposedly very good and pricy top coat. If the base coat fails for any reason you just wasted ?all? of the time and money you spent on this project, and it will most likely fail before the clear would even if it is compatible. There?s nothing wrong with low cost if that?s the objective, but it might pay to stay with a line of products designed to work together.