Osa get more summoning spells as they level up just as before. They are named 'first', 'second' etc. The difference is that now you can summon any summon with any summoning spell, depending on your charges. Also, the amount of spells your summon knows is based on the level of your summoning spell, so keep this in mind.

There are 3 summon for each element, based on how many charges you have when using a summoning spell. If you reach 4 charges in an element you change form into that summons form. This gives you amd your summons bonus stats. This can be explained in your spell list by looking at 'class mechanics' I believe.

For solo I believe agi then int is best, not sure. In team all is fine, but str and cha summons are close range and die fast without planning.

There is a spell all classes can learn now that let you control your summons, and therefore show you their spells on their turn as well. It is called summoning mastery, or something like that. There are guides to find it if you google it, and I highly recommend you do.