Dog Faced and Panthers are not slow growers. They'll put on the majority of their size within the first year. It's not uncommon for people having 2" Panther Groupers and within 9 months, they're 6" - 8".
My philosiphy is "dont buy a fish that you dont have adequate room for, as an adult". People buy tanks and then buy fish that will greatly outgrow their tanks, with the intent to upgrade. Then, when it's time to upgrade, something happens, or they dont have the money, etc. and they're stuck with a fish that becomes stressed, diseased and eventually dies because of being cramped.
And, "they grow relatively slow ecspecially when they are limited by the size of their tank" is a very big misconception in this hobby.
I also did not start out with large tanks, like the ones I have today. My very first saltwater tank was a 12gal. JBJ Nano. Then, I upgraded to a 30, then to a 55 and then to my 200.