Is the Superior Tactics ability able to cause a damage and a retreat from the same die or do I have to choose to apply the ability to gain a retreat and thus forfeit the damage. If I do not have to choose to apply a damage or a retreat and the double sword die can yield both damage and a retreat am I then forced to use the retreat against my opponent or would that be optional. thanks in advance for your comments.

If you commit a result, then that die is bound to the ability triggered. This means that if I roll with Citadel Guards a result, I can choose to:

a) use the result to deal enemy unit one damage or

b) commit the result to trigger Superior Tactics and cause enemy unit to retreat

the options are an alternative, and cannot be cumulated (i.e. one result cannot be used for two different things)

You can find the rules for this in the Reference Guide, pag. 3, under Committing Dice, bullet point 3

The ability is still very nice, as it gives the Citadel Guards a better chance to push an enemy unit off of a space (especially VP) spaces, plus with Pursue it lets them make another attack.

It's nice when you roll two of them, because then you can do a damage, push them back, move up, and attack again. Sometimes retreats are better than straight damage if they prevent the opponent from countering

True. Just imagine you want to remove an enemy unit from a building or a VP banner (or both at the same time): now you have 1/3 chances to succeed.