There is a newest post of theymos on Trust system, that should be added in the Trust section of the topic,
there. In this post, theymos clearly states his opinion that if someone leave wrong negative trust on others, that user should be readily to wait for negative trusts from others as back-and-forth negative trust. Hence, negative trust should be used appropriately at the first time to avoid such uncomfortable situation and other drama on Trust system.
I was always intending to choose a random subset of 100 once more than 100 became eligible. This creates more people who have a credible threat of retaliation: if you give someone negative trust for some stupid reason, you have reason to worry about them or a close friend of theirs negative-trusting you for a similarly stupid reason, if not in this month, then in a future month. I think that it pushes people (without forcing people) toward acting in-line with consensus, so that any retaliation against your sent negative trust always gets the sender excluded definitively.
IMO it'd be nice if in the future it's a subset of 100 among a pool of 250+.