Well the tag is supposed to link together stuff that has similar interest. I would think primates are an umbrella search interest, but if any devs think otherwise, feel free to correct me. If someone were looking for orangutans specifically, they'd type that in the actual search bar and not as a tag.
Maybe dominatrix and femdom shouldn't be linked, but I would think "femdom" and "female-domination" should specifically as one is an abbreviation of the other. Likewise "dominatrix" and "domme" should as abbreviations.
To my knowledge, unofficial tags can't have spaces and any search attempt with a space has a hyphen, so "big ass" automatically becomes "big-ass".
"10-year olds" and "10 year-olds" becomes "10-year-olds" combined automatically so that sort of becomes moot.
Strangely, only official tags have spaces.