Importing cockpits has a well established community precedent, so I'm a-OK with that - why reinvent the wheel after all? As far as importing other bits of texture, that's... trickier, and we may have to fall back on the "case-by-case" situation, as unfortunate as that is.
Certainly, I've used pieces of other people's textures (see the Abydos for a fairly shameless rip of pieces fo the Karnak) and I know a lot of other people have to - it's a great way to add detail and help a model fit into an already established species "look". But in this case, given that we're judging people on the quality of their texture, it becomes a bit iffy, especially if it's from a non Volition ship.
If I were forced to make some kind of definitive statement, I would say that a few details from other ships are fine, as long as they're credited (particularly if they're easily identifiable - you don't have to worry too much about a few pixels if you're grabbing them for the colour palette or something), more liberal sampling is acceptable from canon ships, but excessive sampling will lose you points at least in the technical side of the texture grade.
Basically, be reasonable about it and you'll be fine.