The difference between warrior/paladin and paladin/warrior is that you get your main job at whatever level you've got it at, but your sub job (the second one) is only half that level.

The summons do look pretty cool. Just keep in mind, until you're a really high level summoner, you'll probably want to have white mage as your sub so you can back-up heal. Then you'll start to summon them so they can cast their buffs. It's not until much later that you use them for attacking.

And yes, you get your subjob at 18, you have to do one of two quests in either Valkurm or Buburimu. It's easier to do it in Valkurm.