How do you select a second class?

[Bixby]

Ah thanks, I hadn't realised that. So can you unequip the Paladin soul crystal in order to get the full Gladiator skills and cross-class skills if the situation demands that? Or is a case of once equipped, that's it for ever?

The former. You can unequip a soul crystal to switch to its base class, re-equip it to switch to its job, anytime you could change any other piece of equipment (i.e., not in combat).