Back in the original Final Fantasy VII game, you could use summon materia during any battle as long as you had enough MP. In Final Fantasy VII Remake, I can't find a way to use summons during any battle, only certain pre-defined battles. I would like to be able to use summons during any battle, so that I can capture screenshots of them. Is there a way to unlock the capability of using summons during any battle?
You can summon in regular battles too, it's not just the boss fights.
In order to start the summon gauge you usually need to either get a character to low health (in the red), or stagger an enemy. Once either of those criteria is met the gauge will start filling.
However, there are some situations where certain summons are not available. Based on my experience and research, summons are only available if the battle area you are fighting in is big enough for them to fit in and move around in.
So there will definitely be some battles in tight areas where summons won't fit, so can't be called. The smaller the summon in size, the more likely you will be able to use them.