IMHO, you have two choices:
1) a (relatively) expensive two slalom board choice (but with a perfect wind range coverage) where a 7 sqm. sail with 110-115 liters board (i.e. iSonic 111) is for medium to high wind and 9 sqm. sail with 130-135 liters slalom board (i.e iSonic 133) covers low wind range.
2) one slalom board choice, less expensive (but imho less efficient), where you could buy i.e. a iSonic 122 (2008 model) that, by all account, is a very polyvalent board capable to carry small to large sails (from 7 to 9) with good performances.
Pointing only on a iSonic 133, as suggested above, for using it in 7 sqm. range sails is (always in my opinion) a little restrictive because this board is designed for light to medium winds.