About the piezo, I don't recommend adding series resistance, at least not the series value that is shown by CTS as a protection device. By the time you get even close to damaging the tweeter, you've long ago destroyed the woofer/midrange. So it isn't necessary.

I used to install a very small value resistor in series, but I used it as a fuse and not as a current limiting device. And I was concerned about oscillation when I first considered using them too.

But after nearly 30 years of experience, I can assure you that these are unwarranted measures.

The kinds of circuits that encourage oscillation are unsuitable for use with piezo devices even with the resistor installed. Notice the previous post called "Don't go into clipping and check for back-EMF", where Andy reports that his piezo tweeters "buzz" (oscillate) even with the so-called protective resistors installed.