Do I need isolation footers for my components if I'm already using an isolation rack?

What rack are you using??

Mine. I would call it the Miller Carbon like my turntable but its mostly concrete so I just call it mine: Modular design solid concrete shelves with built-in sand beds, bolted to ABS concrete filled columns, topped with granite. Still need BDR Shelf, Cones, etc. https://www.theanalogdept.com/c_miller.htm
The other rack in the middle is some generic thing I've long since forgotten. This is an old image from 2004. The steel rack btw cost about twice what it cost me to build the much better one.

Here is an easy exercise, to answer your question.
Gather household items: rubber doorstops, protective furniture leg discs, pencil erasers, pill bottles, wooden cubes, different
kinds of tape.

Place them under and on top of: electronics, cables,
ends of power cords, speaker cabinets.  You can even use them in some combinations, together.

Some will improve the sound, some will make it worse.
The piece that improves what you hear in one application, might not work in a different one.

This is a harmless, low cost way to experiment with tuning your system.  The higher the quality of your setup, the easier it will be
to hear changes.

At some point you may want to make an investment in
products designed for these applications.

Make it fun.

One of the best posts in a long time. Well done.

Nemesis, I use Herbie's footers under my amp and streamer-dac....and I use a solidsteel brand rack.

My sandwiches of different density materials works fine for me ….Quartz feet, granite plates,cork plate, bamboo plate, sorbothane sheets, all that in sandwiches seems to controls the resonance and vibrations of my gear that is on my desk...

One day I will try, springs, and crystals beds under the speakers also...

I use whatever LIGO uses. I.e., mass-on-spring. It’s not rocket science, folks. 🚀 Well, maybe it is!