Amazing Job on this Rendering. The Lines are clean and the colors and shading are amazing. Fantastic job combining the two images and adding in the background. Overall this is a beautiful piece of work of two beautiful characters. I can definately, see this hanging on a Batman enthusiasit's wall.
The only downsides that I can see is the fold lines on Harley's Stomach area, I would take the middle ones out and give the bottom ones a downward angle. By the highlights you give and the fact that you can see her abs and belly button it gives the impression that her outfit is skin tight but the fold lines are not matching with that impression. Good thing is that its not so great as to distract away from the rest of the piece, which is amazingly beautiful and well composed.
Yes Harley is roughly 5'7" and Ivy about 5'2", but nobody ever gives her the figure or the legs of a woman that is 5'2" So I just drew them the way I felt like. My only other justification is that Harley is behind Ivy, and at and angle from waist level and not head on. But, nevertheless, good eye, and thanks for the compliment.