So if I have an equip with charge reduction low and another with charge reduction low, does that equal medium? Similarly, low+med=high? Or is it more like med+med=high?
At some point does it just cap? I.e. Any point in having high+med+med+low+low after using the right equips and UVs? This would be really good to know.
It's the same as any other damage, attack speed, or charge reduction bonus. In number form:
Low: 1
Medium: 2
High: 3
Very High: 4
Ultra: 5
Maximum: 6
So yes; Two lows equal a medium, and it caps out at six lows/three mediums/two highs/one very high, one medium/etc. Once the cap has been reached, further bonuses will have no effect.
Certain equipments MIGHT have a higher cap for attack speed or charge reduction, though those are really rare and I never really got to confirm that myself, but in general, that's how bonuses work.