The Rustoleum guys probably would like you to use their enamel, and don't want to suggest a competing product. Quite likely their marketing department censors their replies. More likely, their marketing guys are the only ones permitted to reply.

Rustoleum primer has been almost universally used by "rebuilders" that I've known in my area, although most of the ones I've known very recently have had all the finishes applied by custom shops, so I don't really know what's the latest practice.

Any finish paint, particularly an automotive one, should be labelled with surface preparation requirements. If the finish material doesn't specify a specific surface treatment then it's probably safe to assume that the Rustoleum primer is okay, as long as it's been aged long enough for all the volatiles to evaporate.

You may have to allow for the label on the finish paint coming from "advertising" too, so a brand name there means less than a generic statement about a specific kind of underlayer prep material.

About the only place you're likely to get a more specific answer is from someone who's in the business of painting lots of autos, and can (and will) tell you if they've had a problem. (You're probably more likely to get them to talk about something that didn't work than about how they do it when it does work.) The second fairly good alternative is to paint a couple of test panels and make your own judgement, but this takes time if you're going to see a "finished" test panel. Initial appearance doesn't really tell the whole story, although it may be reassuring enough.

John