I am doing exactly what you are doing. I am also using Postfix and Courier IMAP/POP3.
It is possible to implement soft quotas in Postfix but you need an extra patch called VDA. See here:
http://web.onda.com.br/nadal/
I havent got this far yet though!
I have got postfix installed and implemented the MySQL part with virtual users and its now accepting mail for those virtual users and writing to the filesystem in Maildir format.
GIve me a shout if you need any info on this.
Dan