Bit late to the game here.  But one comment to make: with respect to "collage" type designs like the OP's, they can look less crowded if the background elements are successively faded and/or de-saturated.  This mimics how things look in real life or in photographs.  Stuff up front is sharper and more rich in colour; stuff in the back is less sharp and more faded.  Then objects will look like they are occupying their own space instead of overlapping each other.