It sounds as if you have enough of the colors you already purchased, but what you still need to buy are other colors? If that is the case, then how many ounces are in a skein will make no difference. However, if you do order it from an on-line company, I would call them and ask if they will check the dye lots because every one I've ever looked as explicitly states "dye lots not guaranteed".
The other issue is how many ounces per color are required. If you get 6 oz skeins instead of 7 oz, you could potentially run out of yarn.