Question

when dealing with big equations in redox reactions that have many of the same elements, how do you know which ones to pair?

Answer 1

In RedOx reactions one or more elements give electrons away. The others element(s) accept(s) electrons. So, these two types of elements should be paired.
And when you are trying to balance the RedOx reaction the total quantity of “given” electrons should be equal to the total quantity of “accepted” electrons.