Are all the ship part tiles infinitely stackable? We were ending up with Interceptors with 6 Ion Cannons (adhering to the power supply of course). And are the power sources stackable? Can you stack two or three Nuclear sources, for example, to generate 9 power?
As long as you're obeying the basic rules of shipbuilding, you can put as many of any ship part (Ancient ship parts notwithstanding) on your ship as you wish.
While the effects of ship parts are cumulative (e.g. two power sources would provide twice as much power as one), ship parts themselves can not "stack" infinitely. Each ship class has a very finite amount of space, which is printed on the player card. A typical (e.g. Terran) Interceptor, for example, only has four slots available for ship parts, one of which must be used for a drive. If you want to put a new ship part into one of the spaces, you have to remove the old one (or cover up the pre-printed one).
In other words, only four ship parts can be active on the Interceptor at any time.
So between the drive and (presumably) a power supply, you could have at most two Ion Cannons on an Interceptor.