Choosing additional classes

[HighMarshal]

When would you suggest trying the different classes? When you hit 15 in your first one or should you go all the way to 20 in your first class and only change after you get your mount from the grand company?

[Garlyle]

When would you suggest trying the different classes? When you hit 15 in your first one or should you go all the way to 20 in your first class and only change after you get your mount from the grand company?

Whenever you want, really. You can start trying other classes any time after 10 (The level 10 class quest of your starting class is necessary to swap classes), with access to all but the Heavensward jobs at 15.

The big advantage to testing out other classes is that some of the skills are actually transferrable. Some good, easy to access ones include...

For damage: Straight Shot (Archer 2, attack that boosts crit rate), Raging Strikes (Archer 4, boosts damage), Fracture/Venomous Bite (Marauder/Archer 6, DoT attacks), Internal Release (Pugilist 12, boosts crit rate)
For survival: Foresight (Marauder 2, boosts defense), Feint (Lancer 2, low damage attack that slows), Cure (Conjurer 2), Featherfoot (Pugilist 4, boosts evade), Keen Flurry (Lancer 6, boosts parry), Second Wind (Pugilist 8, instant self heal), Protect (Conjurer 8, perpetual defense buff), Bloodbath (Marauder 8, grants HP drain)

Not all of these will serve you forever, but they're excellent to have around at low levels (pre-Job) and some will still be useful even once you get your Job too.