In theory, Rex with the above combo can put damage on up to 4 ships, as the cannon and primary targets don't necessarily have to be the same ship. Even if they were, there may be two ships within range 1 of that targeted ship.
With that in mind:
A) If the cannon (Ion, for example) hits one target, primary hits a second, Ruthlessness kicks in for each attack, dealing damage to two targets, for a total of four damaged ships; how many of those ships are eligible to be hit by his pilot ability? Just the target, or the target and the spillover target from Ruthlessness (since it's damage coming from the same attack, as Ruthlessness isn't an attack by itself).
B) Since the cannon and primary attacks are separate, if both attacks are on the same ship, at what point do you choose to use the ability? After the attack, or after the attacks are complete?
C) If ships dealt damage by Ruthlessness are treated as separate attacks, are they eligible to declare as the target of his ability?
D) If Rex has more than one Focus token (let's say there's a Fleet Officer nearby, dishing out tokens like candy), can he trigger his ability on two different targets in the same turn (different cannon and primary targets, for example).
I think I've made that sufficiently complicated... It is something I'm curious about though. Rex hitting 4 ships with one volley makes me chuckle enough to want to try it. Being able to turn over a significant number of cards more than usual, though, has be wondering how the timings all work and how those attacks are defined.