The store expert was correct in that self etching primers are old school and today's epoxy primers are much better.
The Ful-Thane 400X is a Nason product which is DuPont's economy line. I strongly suspect that this combination will be fine and they will work well together but if the fact that the "P" sheet for the Ful-Thane doesn't mention compatibility with the DuPont expxy primer bothers you just return the topcoat and go with one of the Chroma single stage urethane topcoats instead.
There are a couple of paint gurus on this board that can probably confirm or correct my suggestions so I would wait for one of them to chime in. Although I'm a Moderator for the body/exterior forum I still don't have all the answers. The fact that both products are in the DuPont family makes me suspect there will be no problems. If one was not a DuPont product I would strongly council against using it. MartinSr or BarryK or BadBob will be able to confirm this though.