You are correct about the fish inch/gallon. It's a very general rule. However, the only answer I can give is "IT DEPENDS"... on
How often you do water changes;
How large (%) is each pwc
Do you have a skimmer sized (or oversized) for the tank;
How often do you feed;
Do you have enough LR
for biological filtration;
Are there enough hiding spaces; overhangs, tunnels so each fish can have it's sleeping spot and territory.
I have 3 tangs in a 6' long tank and wish it was 16' long.
I have a total of 12 fish in my 125:
1 SF tang (Zebrasoma veliferum);
1 YT tang (Zebrasoma flavescens);
1 Blue tang ((Paracanthurus hepatus)
1 Coral Beauty (Centropyge bispinosa)
1 Bicolor Dottyback (Pictichromis paccagnellae)
2 True Percula clowns (Amphiprion percula)
2 Bicolor Blennies Ecsenius bicolor)
2 Spotted (Pajama) Cardinals (Sphaeramia nematoptera)
1 Green Mandarin (Synchiropus splendidus)
You're thinking of 6 tangs in that tank, which may be too many. You can try it and see it sqabbles develop and then be ready to remove one or more and place them in another home.
Don't worry about the mandarin. With 300+ pounds of LR
and a mature tank there will be plenty of pods
for it to be fat and happy.
Wish I had room for that tank though (but I really wish I had the room for something like Matt's 700 gallon tank).
Good plan, go slow and only add a few fish each month as the tank matures.