I will chime in, more about what I think about the 6' rule and the
stock your tank for the stats you have now rather than what you plan to have in the future" rule.
The 6' rule is a very strange one to me. I am not sure where it originated, or who came up with it, but it doesn't seem to make a whole lot of sense. 6' is still vastly smaller than these animals natural swim space. Why 6' is the magic number is beyond me. I have seen tangs in 4'-10' tanks and they all tend to do one of two things. Swim back and forth, or essentially circle the rockwork. Unless the animal is so big that it doesn't even swim from one end of the tank to the other, but rather reaches the tank end to end, I personally fail to understand this restriction. My point it that I think this restriction could/should just as easily be 8' or 10' (or any larger number for that matter). My theory is that 6' tanks are by far a more popular build (talking standard here, not custom), and as such they are the "largest" tank to recommend. It is much easier to recommend and sell a fish if I don't have to have a custom 10' tank built around it. Please understand that this is pure conjecture on my part, based somewhat on all of the readings I have done.
As far as stocking a tank for what you have now instead of what you plan to have, I think there is some merit to that. If you are simply dreaming of a tank in the future to put your fish in without a budget or anything much in place yet, it would probably do you and the fish the most good to hold off. If you do have plans in place, I say go for it. The counter argument for this is that plans can change or fail. I agree to that statement. My counter to that is, plenty of people with large enough tanks have plans change and end up tearing the tank down. My point? Nothing is set in stone. Should we remain out of the hobby because we might not know if we will have said tank for the next 5, 10, 20+ years? IMO, no. I feel the most important thing to remember here is to still keep the fish's best interest (having already taken it from the wild) at heart. Let's say plans on your 500g dream tank fell through and you have a tang outgrowing your current system. It is your responsibility to still do something about it. Be aware, this may require you to damn near dismantle all of your rock work to get that sucker out. IMO it is your duty at this point to find the animal a home. FWIW, I doubt you will have a hard time giving away or selling a large tang (maybe not an achilles due to their difficulty), unless you live in a super remote area.