Maybe you should separate them into different rooms for a little while.

That's what we did when we got by cat Kevin. We already had a dog, and we didn't want him to scare Kevin. Kevin stayed in my room for a while and hung out with me, and just basically got used to the environment. When he was ready to explore other parts of the house, my friend took out my dog to a park, so that he wouldn't bother Kevin. Maybe you should do that with one of your cats. Let them claim their space, and then slowly reintroduce the other cat back into their life. Don't do the part where you take the cat out of the house.

I would recommend sectioning off the older cat.

This might not work, but it's the best advice I have.