I think the best size combo with the 12m is a 9m, the 7m would be too big a jump from the 12m.
Most companies seem to be moving towards 9m as the sweet spot size to mix with a 12m. See how the Boxer came out in 9m, now the R5 is going to have a 9m size. I have a buddy with 04's Slingshots and he likes the 9m the best. Turns fast, depowers great, and is fast but not too fast.
From now on I think I am going for mid AR 11m and smaller, and high AR 12m and up. You could swap between mid AR/high AR for medium sized kite (12-13) depending on what style you prefer. I would say if you are lighter go for high AR for medium size, heavier mid AR. Wakestyle mid AR, boosting high AR. Wind range high AR, turning mid AR. Chop & Waves mid AR, flat water high AR....
My 10m Nemi is an excellent kite for crusing and boosting, its just so fast that doing wakestyle on it is more like crusing for a brusing!