Comma check

I know there is a similar rule with sub claused (when they come before the main clause their is a comma to seperate them).

I've found that sometimes conjunctions can have a different role in a sentence i.e. can be a conjunction or preposition depending on how used.

In this e.g. is since a subordinating conjunction.

Since leaving university, I have worked for the Gap and have been promoted to store manager.

The 'I have worked for the Gap and have been promoted to store manager' is two main claused joined by a conjunction, but it's short to the comma can be missed out. The 'Since leaving university' cannot stand alone so it dependent on the 'I have worked...' to make sense.

Is this correct?

Also what about when I use 'including' i.e. I have worked as a waitress for five years, including three years at Cafe Smith.

Is the comma needed and if so why?