You can use letters and numbers but it is best to use numbers at the end. You should also use leading zeros. For example if you entered a starting Customer Number of CUS-01 you will run into a problem when you reach 100 customers. You would end up with a number like CUT-99. You should use enough leading zeros to handle the largest number you thing you will need. For example a starting Customer Number of CUS-00001 would allow for up to 10,000 customers.
Custom numbering will not apply if you use replication because numbers are not generated automatically and must be entered manually when using replication.