I'm a conjurer soon to have arcanist at 15. I understand it is best to switch to white mage at 30 even though I could switch eg at 40 and have conj as well as WM abilities for 31-40 after doing class quests. The theory being that conj abilities over 30 aren't worth playing as the weaker class and groups can get snippy if you try it. Correct?

Once I hit the expansion - this requires both level 50 and completion of all old world msqs I believe - I can add tank and ranged dps to my repertoire to have some diversity and the dark knight and machinist start at level 30. These classes are at least as powerful as getting old world classes to 30 the hard way though they should be supplemented by getting cross-class skills from old world classes and I should try the old world classes too in case I prefer the play style. Also correct?

Thanks.