Invoice Layout default linked to login

Hi, anyone know how to link the default invoice entry screen to a particular layout?  I have plenty of customers that we either sell stock to or service items so setting the default to the customer won't save me time at all.  If I could link it to a login we can create a separate  logon to enter all of the service invoices from this login and we don't have to change every single email we create?  (any suggestions for a different type of workout would be very welcome as well - we invoice over 200 customers regularly so creating a second account for each customer isn't the most labour effective way of resolving the issue).

Janelle