When the proposed tagging system is implemented, it would only be natural to have it possible for GM to remove tags as well as add them, right?

We agree with the above. adding extra sub-sections for group sizes would not be practical or provide a clean, uncluttered look. Besides, where some consider 3-5 to be small, others might have the numbers be 3-6 or 4-5 or... And some wouldn't just split it to large/small. Some would split it to small-medium-large, or perhaps tiny-small-medium-large-massive. There's simply too many ways to divide such up to have a clear system based on sections. 

If anything, the more logical sub-division would be by RP genre. But just a quick look at the more common genres, and you see the same problem exists there. Too many.
It is, in our opinion, better to wait for the proposed RP tag system to be worked out.