Is mixing RAM brands and speeds a good idea?

It’s a gamble. Motherboards will default to the slowest stick’s speed and try to match timings, but XMP profiles won’t align, so you’ll likely lose performance or get random crashes. Your best bet is to check your motherboard’s QVL and buy the exact same model if adding a stick, or just get a matched kit if buying new. Don’t save $20 to risk random reboots.