Several things:
First (on empires):
Almoravid was essentially correct with his statement as to what an "empire" is. However one must also add that these additional areas must be forcibly annexed. Also empires are not limited to dictatorships or monarchies. Roman Republic and US were (and in the latter could still be seen as) empires.
Second (on # tags):
Pelluid - While initally i proposed 20 tags used, its no longer that high.
Doktarr - Eldin's 8 tag proposal only concerned the Sengoku Jidai period. He said atleast once he was not familiar with the Onin war period which would require additional tags. He also did not say if he was familiar with the Tokugowan era which would require atleast another aditional tag (Chosokabe if it wasn't used before).
Third (on my other proposal):
Doktarr, even at that time their were huge flaws with it. HUGE. It was better than nothing, yes, but two pennies are better than one penny. And I still don't know if it would go off right and somesuch because of the mass changing in-and-out of dynasties tag changes, mass event firings, etc. That's not including the quickness that would likely have occured. Plus i came up with a better, but still quite poor, working for the Onin war, something that wouldn't work this time arounds.
Also another reason I was able to pull it off was that there were no capital moves and all provinces were named after their head province capital. That helped. This time around it won't work at all because they all have different capital provinces.
Conclusion:
The dynamics that allowed for 6 tags aren't there and it would far more unfair to limit them to that or even the 8 (because as said, Eldin's proposal only revolved around the Sengoku Jidai period. You can't ingore the rest of history, unless you're going to do the same for HRE and just focus on a brief period).
Also even with 15 tags (hell even 20, but i'm not asking for 20), this is a vast simplication. After the end of the Sengoku Jidai when many clans were destoryed, there were estimated to be around 250 left, give or take 100. All those clans held territory. Just like HRE, most of them are small and unworthy of being modeled on any level, so I don't bother. So yea this is a simplified setup of Japan.