A mixer will have multiple channels for your multiple inputs.
Each channel will have a trim pot (gain knob) for adjusting the input level.
(Sidenote: some channels may not have a trim, those are line-level inputs, not mic-level inputs, and they are meant for things that already at "line-level")
You set your gain as you need to for each channel, nothing more or less. The only real standard is "don't clip the channel." (Unless it sounds good... but really, for what you're doing, don't clip anything)
You generally don't want to be making crucial mix decisions in the field. Ideally, you will be using direct-outs from the mixer, but you can also utilize the various bussing options (auxes, sub-groups, matrices, left and right channels, etc.) to isolate your signals as much as possible.
Those outputs are then generally sent to a multi-track recorder which captures the individual sources for use later. The actual mix is postponed, just as the video edit isn't done on the fly.
That said, you absolutely can (and may have to), make some final choices while shooting. This may be because you have a limited number of recording inputs, or because your mixer doesn't have enough outputs. Whatever the reason, it means you'll have to do some mixing in the field.
In those situations, you just do the best you can. Use your ears and mix the signals in the best way you know how. Consider not only what you are hearing, but the tools that can be used later as well, along with what you think the finished product should sound like. Hopefully you're only having to mix a couple tracks together, so you'll only need to worry about the balance of say, two things, as opposed to thirty things. Just consider that small piece in the broader perspective of the full puzzle.