Yes, you did choose the right starter class to unlock Summoner. After completing your level 10 Arcanist class quest, you will unlock the Armory system and your guild master will inform you that you may no experiment with other classes.

Once the Armory system is unlocked, you can literally start joining whatever other battle, magic, crafting or gathering guild you would like! For example, you can join the Conjurers Guild and start unlocking healer actions like Protect and Cure. You can join any crafting guild you would like and start crafting items for yourself and selling them to other players (later on after you unlock the Retainer system).

You will also want to familiarize yourself with the gear set system. This is found in your Character window near your main equipped weapon/tool and it allows you to create sets of equipped gear per class you play and quickly swap between your unlocked classes!