for just increasing damage while leveling, I'd level pugilist until you get internal release. That's an ability you'll use all the time, even after hitting level 60.

You can get the other damage increasing cross class abilities from the other jobs if you want, but I wouldn't bother, as once you become a warrior at level 30, you can only take cross class skills from pugilist or gladiator anyhow.

You also likely will want to level Gladiator to at least 22 sooner than later for provoke. It's an absolute necessity at higher levels, and still pretty useful at the lower levels for those times you're grouped with dps who love to hit anything you're not hitting. (never happens!)