Can I Fuse Calyrex?

Open up the Main Menu by pressing the X button. To rejoin them, click the Reigns of Units in your bag while Calyrex and either Glastrier or Spectrier are in your party. You should then be able to fuse them again.

Can you switch Calyrex forms?

In the story, you actually capture Calyrex in this form. You can change forms by fusing using the Reins of Unity item. You need both Calyrex and the relevant Pokémon to change the form and you need a spare slot in your team to unfuse them.

Can you combine Calyrex and Spectrier?

Once the two are split up, you can merge Calyrex with either Glastrier or Spectrier again by using the Reins of unity on it while both Pokémon are in your party.