Bards should not interrupt others' leveling experiences. If no one is in the zone or no one is being interrupted, it doesn't matter how many mobs they are killing at once. This is basically exactly what the GMs say about the issue.
If the bard does end up interrupting someone's leveling due to high zone population or just pulling from a camp someone has already occupied, the bard and the interrupted person need to work something out. If they can't, the GMs will get involved. That's basically the rule.
Someone just made a thread about bards that's like 5 posts below this one:
http://www.project1999.com/forums/sh...d.php?t=175919