Up to lvl 30, just about any DD can tank, or a duo tank. After lvl30, jobs start to specialise more, mob's hit a lot harder, and 'more' is needed.
Up to level 30 tho, its good for pty's to have maybe a little dnc, cor, and brd in the pty's if the main job's are not there.
/DNC can keep the pty Drain Samba'd
/COR can get a little extra exp from Cor's roll, then from 22-32, you can give a nice static acc boost.
/BRD for a minuet.
Garliage Citadel + is when you really need to have the 'proper' support jobs, and for WAR, /NIN and /SAM are the two you will need.
a Combination of WAR/NIN WAR/NIN, NIN NIN, NIN WAR/NIN can duo tank mobs perfectly until a NIN receives Utsusemi:NI @ 37.
Taking a break.