I understand what your point is, Mister Ed. And I agree there's only so many ways you can build, say, the cockpit of Luke's Landspeeder, without making something look like it was ripped off an official set. But If you look at the samples that Don posted, there's a very distinct lack of effort, like this one:
Exchanging the engine nacelles from Grievous' starfighter with the ones from a Naboo starfighter isn't exactly rocket surgery. Neither is slapping on a tail taken from a droid trifighter set. Those are just official sets reattached together.
If you want to build a y-wing cockpit into your hybridfighter, and end up building one that looks exactly like an official set then i say rebuild or rethink the design and build something that's your own. your peers are going to want to see something original and will spot cheap shortcuts.
The best advice I can give anyone attempting this contest is this: think of two ships you want to mash together, and build them from the ground up. do not use official sets as guides. don't even have them in sight. do not use official set instructions as guides. sit down with a pile of bricks and build. that will be the easiest way to avoid what we, the contest admins, are trying to avoid seeing.