Acala, Sol, and Brynhildr make an excellent base to build a Fire team around. You should be able to do most if not all content with that. Make sure you max Apocalypse when the revival comes up. But really, you should max all of those dual-element eidolons anyway to keep your options open.

Gaia is being hyped because she's the direct counter for this event. Sol is more versatile and desirable in teams of an arbitrary element. For Fire, you'd want Amaterasu to be 'optimal', but there's no use dwelling on that.