Since you're working with a small space, choose plants with a very long season of bloom (peonies and glads are out). Pollinators prefer masses of the same plant so plant in groups. Columbines bloom when the hummingbirds first arrive in spring and the poor little guys are famished. Zinnias plus phlox would be great for summer for both hummers and butterflies. In northern VA I rarely see a hummer after mid September so fall bloom isn't a concern for them. Zinnias bloom until frost to feed butterflies into fall. So I would use phlox and zinnias edged with columbine.