Dual Boot Monterey and Yosemite on Mac Pro Late 2013 ?

I currently have a late 2013 Mac Pro that I've been holding back on upgrading. My Apple SSD is split into two HFS+ partitions, one Yosemite and one High Sierra. I'd like to reformat the High Sierra partition as APFS and install Monterey on it, while keeping the Yosemite partition intact (I need to run FCP7 and Adobe Encore for another year or two). I've cloned both partitions to a backup. Two questions:

1) Can I just reformat the High Sierra partition as APFS and install Monterey on it or do I need to reformat the entire SSD as APFS, partition the SSD in two with one APFS partition containing the Monterey installation and one HFS+ partition containing my Yosemite clone?

2) When Monterey is installed, I understand that the firmware will be updated. Will this cause problems running Yosemite, disable Yosemite entirely, or will there be no adverse effects? I realize that the firmware update is a one-way trip and I can't go back.

Answers and advice would be greatly appreciated. Thank you.