I bought a couple of Feliway difusers at first. They are kind of spendy but they did seem to help calm everyone down.

I'm currrently employing the "time-out" system with the two that have difficulty (Sidney and new guy Strauss - Strauss thinks that he should be top-cat and Sid doesn't agree with him). If one of them is getting too stressed out - they get locked in their room for 10 minutes.

Sid was starting to get more skittish and nervous so I've been trying to give him extra love and attention.

Also, you might consider getting a collar with a bell for Lydia, this way at least Briggs can be warned.

Good luck, I know its frustrating.