the cost is zero.
the wording does not limit the number of pilot skills you can have.
so, which skills would you saddle an a-wing with?

I assume you are speaking of the "A-Wing Test Pilot" upgrade. If so, it is a ship upgrade, so only one can be installed, in essence giving you up to 2 EPT's (the card also states in must be 2 different EPT's). This will be great with green Squad pilots along with the "Chardaan Refit" and then install PTL and possibly VI and all for 21 squad points! I mean that's a dual action 5 skill pilot for 21 pts!!! On Tycho, Test Pilot with PTL and Marksmanship could be fun. What I am curious about are any new EPT's that may be coming soon!!!

Not really a rules question, and being discussed already here

http://community.fantasyflightgames.com/index.php?/topic/101737-double-epts/

As well as in the main Rebel Aces!! Thread.