Category keywords don't necessarily have to fit a logical hierarchy, but they still have to be semantically related. They have to be "about" the same thing in some way. The semantic relevance of keywords is determined by Google, not by us, but even still, our sites have to make sense for people. So we let Google do the first sorting and then we use our human sense to deal with what's left.
I know these are just examples from your imagination, but let me use them for real to illustrate my point.
If I use "rock drums" as my seed keywords in Market Samurai, "rock albums download" does not come up as a related term. This isn't really too surprising. It's just way too much of a stretch to think that a site about rock drums would also be about "rock albums download." The two terms are pretty specific and they don't really relate except on some very vague "music" level and that is too vague. I'm not saying that your category keywords necessarily have to come from your theme keywords as seeds, but they do need to show up in the same list somehow.
So first Google has to see them as related in some way and then you have to see them as related in some way. It doesn't have to be a hierarchical relationship, but the relationship of the words does have to make sense both to Google and to people.