pea, i don't know any official lines, but this is what i think about it:
brackets - avoid if possible. if the thing you want to put into brackets is important, then don't put it into brackets. if it isn't important, leave it out altogether.
footnotes - anything that is not strictly helpful for the point i am trying to make in a section/chapter, but is still an important overall input. it's important but if left in the main text it would distract from the point i'm making and would make the text difficult to read.
however i've much more experience writing in german than in english, and styles/conventions might be different.