First off Aileen, let me say that it is a pretty good arrangement done with leftovers! I'm sure that if you had more sunflowers, you would of made it more proportional.
I have dealt with many leftover flowers and found that if you keep the arrangement low, it prevents all the proportion/gaps problems from occuring (especially since you don't really have much choice when dealing with leftovers). You still have to watch the colors (I agree that the white is tricky to use with bright colors).
In the case of the arrangement you made, I really like the euchalyptus collar you have that the base, and I would of gathered the rest of the flowers in a posy and inserted them that way. Then you fill in gaps with greenery (which always seems to be on the floor anyways...).
Athough it needs a little tweeking, I still think it's a very good base arrangement.
Julie