Creating more selective communities with stricter guidelines seems to be the wrong approach to dealing with trolls, especially based on the scenario pg lays out. As pg says, the larger a community grows, the easier it is for trolls to be accepted and the harder it is to mod (prune) the community. It seems that critical mass is just when the pruning community becomes smaller than the trolling community, and that by creating new communities with more stringent rules, you are just delaying the date (hopefully indefinitely) when trolls come in. I understand that the yc community is special, but even the comments here show that it won't work for much longer as the community grows beyond yc. To answer the question of "Will it scale?", I think its already a no.

If you've ever hung out with a lot of girls (from a guy's perspective), you can beging to understand the trolling community. 1 on 1 with a girl and you can get intellectual conversation, but the minute 3 or 4 girls get together, they start talking about clothes, guys, dramas, and all other stuff that just isn't interesting. I bet girls see it the same with guys too (5 guys together = WoW, DOTA, girls or crude jokes).

Solutions? Keep it small. This fails to keep in line with the existing goals of news.yc (advertising for existing startups, attracting smart people, etc). Another solution? Maybe try to preserve the small community feel as the site gets bigger. One way to do this would be to use user upvotes and downvotes as community boundaries for each user - ie: making their community presence only to those who rate them up, and to make their personal community those who they rate up. This is speculative at best, though.