I think 7 is a round, very powerful number. A full coven. You know Daphne, Tracey, Fleur, Tonks, Gabrielle, Luna, and Hermione. Seven witches making a full coven centered on one Wizard, Harry.
I like this idea with one exception. Gabrielle is a little girl so she should be excluded/ Replace her with one of the female Gryffindor chasers and I would endorse the idea.