While I agree that the OP should have added to the other RDM Tank thread, and thus bumping this topic is counter intuitive, I'm going to anyways, cause meh.
A few points.
A: While I cannot comment on XI, due to never having played it, outside Tactics Advanced, which used a weapon based growth system much like XIV admittedly, Red Mage has been depicted as a sub par warrior that could do subpar magic, the two halves supposedly making a whole. They used swords and rods and Staves and shields and lighter armour. Also, Fencers can use shields. Or did you think the phrase "Swashbuckler" used the word "Buckler" Ironically? I'd totally give the Red-mage a rapier in XIV, but that doesn't mean they need to do it to stay true to FF. ((On the other hand they are descended from D&D bards, which frequently are depicted with a rapier, so...))
B: We have three tanks and that's fair representation of the role because you choose to split up the seven DPS into four different categories? What if I split off Warrior, as it is a HP and offence based Tank that excels as an Off tank and therefore isn't quite the same? What if I decided that the RDM being a Mage and therefore a DoM counted it as a separate category that we have ZERO entrants for, matching your splitting of the ranged DPS into physical and Magical?
C: and I flip sides for this one. Tank is probably the one job I wouldn't peg a classic Red Mage for. It was the one role that you either do or do not. I mean, raise your hand if you think Slapping Cover on your Red mage is a good idea in V. I don't think Red mage CAN'T be a Tank. There are just better options.