The Keith Ruston example has some good principles at work that make it mesh a bit better than the OP's...
- The Ruston one has the characters interacting with the background/structure, so they tie in better to the image. That said, the Space Invaders monster looks a bit plopped in there, but he always does.
- The names of games... lose them. They don't blend well or mesh. That tiny 1941 grabs my eye far too much. If you like 1941 that much, try to get the plane art from one of the various 19XX titles. As much as a plane in space would look weird, it would look MUCH better IMO.
- No full side art from games, just take elements. The composition of a full piece of side art ignores all of the other elements, whereas when you pick just a part of the art, you can select images that blend together - something the Ruston one does fairly well.
- Art Styles... not all of the art is done with the same style etc - so they tend to look more like a collage than a coherent piece of art. This is my biggest hinderance to designing any multi-cade art - lots of ideas, but making them look like they were drawn together is tricky without actually redrawing them.
That said... art is subjective too... if its for you, go with what feels good. There is definitely some great side art out there, and plenty of examples to follow... your basis on the Ralston one is good, as it is one of the better ones I have seen. I hope this progresses well for you, as i love seeing someone make something with their own twist when possible.