Sometimes our symptoms do deviate from the standard. It is possible but rare to develop the psoriatic arthritis prior to seeing psoriasis. While all of that is true, I strongly believe that if a diagnosis does not feel right, you should get a second opinion. I would also think that you may want to start with a milder medication and then see how you respond. Enbrel is a drug that they usually work up to, not start out with. I would ask about starting on something for inflammation, like relafen, and then maybe plaquenil. If that does not control the symptoms, then Enbrel might be the next step.
Why is it that you don't believe your doctors diagnosis? Even if your symptoms are a-typical I'm sure there is a reason he gave you this diagnosis. Was it a Rhuematologist? I have Psoriatic Arthritis myself so if you have any questions let me know. I'm very interested in hearing your situation a little more detailed.