1. Should I have a unique, auto-enter serial key Field for each unique table in the database?
Not a bad idea and it does no harm. You'll only need such a field when linking a record in that table to child records in another table.
2. Am I violating the FM laws of nature by mixing alphanumerics for the auto-enter serial (NB: each Field is identified as a Text field).
No, but it's usually not the best approach. Any time you add "meaning" to your serial number as stored in the serial number field, you set yourself up for needing to edit that part of the number at a future point in time and that breaks your relationship.
A better approach:
Store your "letters" in a text field.
Set up your serial number field as a simple auto-entered serial number.
Link tables only via the serial number field. (Don't also include the text field--except in special use cases where the relationship is used to find data not create new records).
For purposes of display, sorts, and finding records, define a calculation field like this: LetterField & SerialNumberField
Place that field on your reports and data entry layouts where you need to see the "combined" serial number.
I agree with Phil (usually do, but not always ;) ),
I opt to mix letters and numbers anyway as a personal shortcut (typically prefixing my serials so I can tell them apart quickly)
WS-000123, WS-000124 for weight sheets... just a personal preference.
This can also be handy when you have travelling copies of your dbase:
Desktop serials start with D-, LaptopA serials start with LA-, etc.
It helps you to know which records to check for updates on after a business trip.
If you choose to mix letter and numbers...make ABSOLUTELY sure that your serial field is text.