The library tagging system in use almost universally, that IMSLP's system is based on, requires tagging according to title or subtitle. Nonet is not in the title or subtitle, so unfortunately, that isn't appropriate. In addition, only the accompaniment would be 9 players, and there is a total of 14 performers here. It shouldn't be coming up on a list of pieces that can be performed by 9 players.
I have re-tagged it for individual instruments, but this will take a while to come through the system, since new instrumentation categories have to be manually put into the system the first time they are used. Until then, it will show as "Unknown tag".
We have discussed other opera categories, and have added a few in the last year or so, so I will bring the "chamber opera" suggestion up with Davydov, who is the leader of the tagging team.
Incidentally, we have a forum for "Categorization/Standardization" so I will wait a few days so everyone sees this, and then I will move this topic over there.