The only requirement is a coherent story. If those sidechapters tell things that could or even should have been told in the main story, then you don't have a coherent story.
I think most of the points up above were things I missed in the main story. I don't know what you precisely mean with "sidechapters" or what you would plan to tell in them, but for now I could only imagine the death of Sixskins to be a "sidechapter" IF it was NOT done by Harry. And if that doesn't even have a four figure word count, it's more of a "sidescene" to be added somewhere in a chapter. For example, at the top like you do now, though so far that seems to be more of a place for Harry's earlier history.
If with "sidechapters" you mean chapters completely from the point of view of different characters, something like the books, then by all means. Though then they aren't really sidechapters but just the way of telling the story. Which btw you already did, because as far as I remember you have already had Sixskins and Jon as main points of view so far. (And then you've perhaps even more reason to put the Sixskins chapters together in one chapter.)