Okay, then I'll use ispconfig database tables. And I agree with you that there won't be the need of an additional table.
I'll also patch the code to enable changing the quota value directly in ispconfig for webs and resellers. That means the quota will be calculated per web, not per database. I think this should be sufficient.
One question is left: I have to add the new fields to the sql table. Therefore my first idea was to patch the sql script responsible for setting up the database when installing. But: I don't know what I have to do to make it compatible when someone upgrades... Please give me a hint