Edited: I got the wrong impression from reading too quickly. Corrected comment:
If needed we can choose a different level than 1000 karma, and change it over time in response to experience, so it's a flexible system.
However, I'm not certain the idea itself is sound. I don't have the feeling that mutual upvoting by new users is a real problem that needs solving. Can you give links to example comments where you think the proposed rule would have helped?
The recent implementation of a -5 karma penalty for replying to comments that are at -3 or below has clearly met with some disagreement and controversy. See http://lesswrong.com/r/discussion/lw/eb9/meta_karma_for_last_30_days/7aon . However, at the same time, it seems that Eliezer's observation that trolling and related problems have over time gotten worse here may be correct. It may be that this an inevitable consequence of growth, but it may be that it can be handled or reduced with some solution or set of solutions. I'm starting this discussion thread for people to propose possible solutions. To minimize anchoring bias and related problems, I'm not going to include my ideas in this header but in a comment below. People should think about the problem before reading proposed solutions (again to minimize anchoring issues).