I thought the Coreopsis was a little taller. I guess not. But you could plant some low flowering plants in front of the celosia, as well. I have some low growing creeper plants that spill down over the rock wall in front of our house that are beautiful in the late summer. One batch has tiny purple flowers, the others are white. Can't think of their name right now, though.
I think we had some of those purple and white flowers, but I don't remember their name, either! They spread nicely - Mom got them for the rock garden in front. I'd know 'em if I saw 'em...I'm sure there must be some in the catalog.