Any limitation with free-sync and high refresh rates on multiple monitors?

Current OS is xubunut 18.04
Since nvidia is now allowing the use a free-sync on there newer video cards i was considering getting a new monitor (my original plan was to get a used AMD card (470/480/570/580/590/vega/navi?) when the next LTS release of my OS with all the nice recent driver upgrades)
Before I do i was wondering if there are any pesky limitations when using multiple monitors as to how refresh rates can be mixed
Can i use a fixed 60Hz screen and a variable 144hz refresh rate at the same time? assuming i can even use variable refresh rates with dual monitors in the 1st place or would it require two matching monitors

I know it requires the 418 driver, but i expect it to be in the graphics driver ppa soon