Read through wall of texts.
Quality issues of huge sets? How are you sure that limiting the number of diffs in one set could increase the overall quality of the map? The two stuffs are pretty irrelevant. Maybe you have to give rational and persuasive deductions.
What could improve the quality of maps should be the modding process in fact. Unrealistic fancy on such number restriction criteria on improving quality is not tenable at all.
Repeatedly, if the map lacks of quality (even not because of it's huge), else if the mapper meets difficulties on ranking such huge mapsets, they will spontaneously reduce the size of their maps, by themselves. And if they stick to their huge sets, they will just undergo a hard time for ranking them. I do not see any meaning of a new rule or guideline.
Dislike PP maps? At first I have to say, not all the huge sets are pp maps. So talking about PP maps under this criteria is already partial. If you are not satisfied with pp maps, you could just stop playing them. No one forces you to play that maps. And if you want to see more non-pp maps, why not just map them by yourself? Again, not all the people are against the pp maps, by advocating this rule or guideline under a reason related to PP stuffs, you are also jeopardizing the recreation of other players.
Worried about too similar difficulties in one set? Firstly I have to say "similar" is pretty subjective. How you define similar? Styles? How you define "style" then? Let's compromise, assuming there are several "similar" difficulties in one map. By limiting the number of diffs, people will stop making similar difficulties? No, they will just make a new set with difficulties removed from former sets because of that limitation. And that will make the case even worse. If you really want to see a more various style of maps, the best way is to join in the mapping and create your own maps, or, giving appropriate and pertinent suggestions during modding period of the map.