Should I pick a dragon for its large troop capacity in Dragonfire?

No, never. It's one of the most common sorting traps.

Troop capacity (≈5,000-6,090 per dragon) is your fuel and durability, and yes, you should fill to the max. But it is not a selection criterion for putting a dragon in an army.

What makes a dragon good is its Order, its ⭐ Habits, its aura (if it's the Center), its green affinity for the type, and its contribution to the army's combo. A big troop tank on a dragon with no good kit just soaks longer while doing little.

The order of operations: you select the dragons first (for their effects and affinities), you choose the troop type next, then you fill troops to the max. Capacity comes in at the very end, as a setting — not when deciding who fights.