I will say that you can successfully do cardinals in a community tank but it can be a PITA. I currently keep mine in less than ideal water conditions (ph from tap is 8.2, and gh is over 400 ppm) I do keep my temp on the high side around 82 F for a lot of my fish but everyone seems to be doing well.
Cardinals like to get ick and they like to get it bad, I've had a few times that they were the only thing is my 75 gallon to get it and I lost a few to it. Besides that though they seem to be used to the water and doing alright though I feel like their growth is very stunted and I'll most likely be going the RO/DI route soon.