[Serusers] Username conventions with Asterisk as voicemail ???

I'm looking for pointers, best practices or experiences from SER users who are using Asterisk for voicemail-only in their SER setup, especially when it comes to planning username/extension/mailbox naming schemes. This is for a large-scale deployment (10K+ users) so I want to get it right the first time. Many of my users will have DIDs tied to their account while others will not. Are there any advantages/disadvantages to having alpha-numeric SER usernames and using aliases for DID numbers? I'm not sure if it matters, but we will be using a third party PSTN provider and will need to support DIDs from multiple countries. Also, are there any scripts available to automatically create an Asterisk voicemail box when a new user is added to SER? As always, your input is greatly appreciated! - Daryl