Does the light effect both types of bacteria or only one?
only one
For example if you ran the tubing coil on the outside of a pvc tube exposed to light, but the bioball reaction chamber was inside the opaque pvc, would this be ok?
this is generally how its done, this is the order
some even use the tubing to block the light from the rest of the bio chamber
Or would the light harm the coil bacteria too that remove oxygen? Or is it the other way around maybe, and the bioball chamber could be exposed to light but the coil should be hidden.
indirectly light is bad for the entire coil, the reasons just change.
EX the coil. its narrow, the nitrate is not low yet, the oxygen is low and co2 is high, add light and you get algae which will clog the coil
EX the bio chamber. though the nitrate is low algae will still grow, it takes that co2 and re adds oxygen, which effects the bacteria, but the bacteria are also light sensitive.
The other question is what is the radio of tube inner diameter to required coil length?
its complex
larger diameters require a much longer length, not only is the ratio of surface area to water different but the bacteria only grow on the surface of the tubing which means your water needs to move further to help it work
For a larger tank you could run multiple small coils or a few coils with larger tubing and I'm curious which would be more compact and easy to deal with (since with multiple small coils you have multiple connections and more hassle/complexity).
generally its always more effecient to run more than 1 coil connected to the same bio chamber. the only significant hassle is when you are cycling the unit. how many you run, and how large a coil you use is dependant on system volume and nitrate rise.