You can get gels in all sorts or sizes, including wide rolls.
Gels are fine, but bear in mind that if you fit say a 0.6 (2 stop) gel you will also reduce the effective power of the modelling light to a quarter, which will render it virtually useless. When you need to reduce the power of just one head, that is usually the best (only) way of doing it, but if you want the power reduction to apply to all heads, it's easier to fit a ND filter to the lens - assuming that the lens will still autofocus with it on, which good quality lenses on decent cameras will normally do.
Too much power can be a mistake, especially when aggravated by not having enough power adjustment on the flash. Most people, most of the time, are better off with lower powered flash heads - it's easy to increase the ISO by 1 or 2 stops if required than it is to reduce the effective power of the flash.