Star Wars ships have always had logical and continuity inconsistencies. They're fantasy vehicles. Do the Falcon interiors match the exteriors? No. Is there a reason why a Twin Pod Cloud Car has twin pods? Eh, probably not. Does R2 fit well in the Jedi starfighter? Not really.
The issue with the extra engine on Poe's X-Wing comes down to whether it's needed for the story and it makes some kind of internal sense. Is it there just to look cool? Why does Poe need a "boost"? In the Empire magazine picture, the engine exhaust is a different color than the pink glow from the X-Wing's engines, so it seems it's doing more than just adding extra speed or power. The ship is flying directly at the new star destroyer, so maybe it provides some sort of unique thrust that allows Poe to penetrate the star destroyer's shields. Who knows. It all comes down to the story. If people like the story, it doesn't matter how goofy the rationale for a strap-on booster is. If the story sucks, people will say the booster ruined their childhoods.