Hi Michael,
The "uid" field is a unique 7-character string for each entry in the lm_users table. Certain parts of LMP rely on this being 7 characters, and the length, contents and existence may change in future updates. To identify LMP users I recommend using the auto-incrementing "id" field instead as it's unlikely to ever change. An option to use LMP with external userbases is to set up a synchronization script that runs daily before LMP's dailymail to import or delete external users to and from lm_users.
Regards