They also need to be installed in the right rotational position. If not, they can hit sections of the frame.
7 coils vs 9 coils means they will fit, but they are stiffer. If you had the original coils you will find they ride softer than what you are putting in the car. That's okay. It usually will also improve handling with fewer coils. If you look at the pound per inch rating you will find the original are probably a smaller pound number.
Now having said that, depending on the coil wire itself, there can be some varations as well. So a spring with fewer coils, but different coil wire MIGHT come out about the same. Again compare inch pound ratings. That will tell you what you need to know.