The primals are considered evil by the humans and garleans. This is because they are the gods of the beastmen tribes that they fight against. The beastmen tribes summoned them as a way to defend themselves and the aether that people are taking from the planet. So the primals being good or evil is just a matter of perspective.

As far as summoner goes, we wont know how exactley how we obtain them. I am of the thought they will be housed. In equippable gemstones like in ffix. Perhaps each job quest gives you another gemstone to use (e.g. In ffix ramuh was housed in the peridot stone)

We are going to house the essence of primals in a lesser being. Yoshi has already stated that carby is arcanist exclusive, however changing to summoner will grant access to other pets, essences of ifrit and garuda having already been confirmed.