You will want an illu for in dungeons, cause illu gets an aoe mp-regen skill
samurai can tank yeah, though holding aggro is sometimes bit harder cause you only gain aggro on attacks, and with a good mage that is mostly less then the mage does.
Even though dragon knight(DK) is on the tanking branch it isnt really a tank. At 70+ its pretty hard to keep aggro as a normal tank that has aggro gaining skills, DK on the other hand doenst have a class specific aggro skill making it even harder to keep aggro on it.
In theory you should be able to go tank/p-dps/m-dps but for that class but in real you wont see someone tanking a dungeon on a dragon knight.
Main classes are important to have, but you should lvl up all other classes as well. Be it for certificates or the class score (total score of lvled classes giving stat boost). There is enough time to do this though, cause at higher lvls the exp you need to lvl up is alot less then the cp you need so its easy to keep your main classes maxed and work on lower lvl classes.