When you have a smaller weapon like a knife fighting a long weapon like a scimitar, footwork is far more important than bladework. If you are using the knife, and are staying out at a range of 2-4 feet where the scimitar wielder can make a solid strike you would be a fool to try to parry it on the swing. The key is to rapidly close into in-fighting range where you can trip up your opponent, and keep them from effectively using their weapon while you have room to use your smaller one. You can do this by circling your opponent so they have to turn before they can strike, or by staying far back so they have to rush to strike, and then close quickly to get inside of their range when they make their move.
You need to be constantly aware of where the opponent's blade is and not be there when it is about to hit you.
You CAN redirect a large blade with a smaller one, but it is very difficult, and you have to be strong, fast, and precise to do it. On the other hand, nothing gives you motivation like a 2 and half feet of sharp steel coming at you very fast.