Focus Fire in combat?

Often yes, but it depends on the situation like anything else. If you have a good chance of taking out an important cap or titan do it.

Focus fire is useful against bigger threats but be aware of the damage types, not every ship is good against big targets so they are maybe better off killing the ships they counter, as they waste huge damage due to their damage types weaknesses. But if his titan is retreating and has 200 health I would target it even with my protevs and trade ships LOL if it would be possible.

Sometimes you may want to avoid focusing fire so that shield mitigation doesn't reset.  This is especially the case if you want to use a high-damage ability, such as an overcharged snipe from the rebel titan.  If you focus fire the target, it does 35% damage instead of 85% damage when taking shield mitigation into account, which means that more than half the damage is wasted if you focus fire before opening up with a snipe.

Focus firing should be used with care...be wary of overseers or other forms of support that will nullify your efforts to focus fire...in general you should only focus fire when such an act will actually succeed....there's no "partial credit" for getting a titan to 10% health...

Lots of games will give you the experience to develop an intuition about these things....you just need to know when immediately focusing on a ship will work, and when you need to kill the other stuff around it instead....