Just a couple quick noobie questions about flashing roms

There has to be a way to tell how stable a Rom will be on your phone...

I had read about wanting the firmware versions to match? What about radios?
Also, how do I know if my phone is lollipop vs. kitkat? Isn't that important if you are S-On?
I'm asking because right now, I'm just booting Roms, hoping they'll work, just begging for a brick.

Sorry if this has been asked to death, maybe my search abilities are lacking. My reading comprehension is not, so links are always appreciated!
Thank you!