You want to get gauge with whatever needle it takes to do that or the sweater will be too big (or small, but if you could get gauge on a 7 and used an 8 the result would be too big, not too small). You could possibly drop down a size (like make a medium instead of a large, do the math based on your swatch and see what you get) with the 8. It isn't much of a change, so the fabric might not be too loose. Personally I recommend swatching and using the needle you need to give you gauge even if it means buying a new needle. You could possibly knit tight and get gauge, but fooling with your natural tension is fraught with danger IMO.