Question about installing Yosemite on SSD

I have an SSD 1TB drive running Yosemite in a 2009 Mac Pro (my favorite of several Macs). Late last year I noticed that something had gone wrong with the OS. It was missing Safari and System Preferences! Never in my 35 years of Macs have I seen anything like that!

I made a boot disk and installed Yosemite on a wiped HDD, and migrated all the stuff on the above drive over and that works fine. Now I'd like to wipe the SSD and copy the contents of the HDD over to it to get the speed back.

So my question is this: what format should I use, if it matters, on the SSD? I've read here that APFS is best for SSD drives, but would it be compatible with an old OS like Yosemite?

Thanks in advance,

Bob