When you get your job crystal to unlock Paladin, you'll only be able to cross-class skills from Marauder and Conjurer.

I use Cure, Protect and Stoneskin from Conjurer, and Foresight and Bloodbath from Marauder.

There's an argument to be made for using DPS cross-class skills, but in my opinion, the job of the tank is to use their entire arsenal to protect the team in a fight. Sometimes that extra 300HP from a paltry tank heal can mean the DPS have enough in their reserves to get the final few casts off and finish a boss. Similarly, if a healer is too preoccupied to protect people after a raise (e.g. in titan), whacking a protect out can be a huge help and take that little bit off of the healer's plate. Stoneskin is great when your melee DPS are taking damage; you can give them an extra 10% EHP with each cast.

Easily the most useful skill is Foresight, and I'd say Bloodbath and Stoneskin are second equal.