Copyright Issue and Substantial Similarity

I've been asked to create several images to look like ships from various science fiction series. I've read a bit about copyright laws, and I know there's a thing called substantial similarity, but I'm unclear as to how this works. I'm not actually copying the source work, but I'll be using it as a reference. I've been given free license for style, so if I do them in, say, pixel art, would that constitute enough of a change?

To cite an example: "Game Dev Tycoon" uses obvious representations of the Xbox, Wii, etc., but somehow has gotten away with it.

Any help or resources anyone could point out would be appreciated.