Nostalrius Begins - Quality wow vanilla realm (1.12)

Can I cast (Let's say Rejuv) on myself?

Yes:

/target Me
/cast Rejuvenation(Rank X)

But, would it be possible to make a macro where when targeting a friendly, I Rejuv them, but when targeting nothing at all, I target myself and Rejuv? I know I could go into interface and check the box "Auto self cast" but if I do that it screws up my addon.

Thanks

There are simple auto selfcast addons which Schoultz do the trick!

Which addon does the normal "setting option" interfere with?

I don't understand how using the simple auto-self-cast feature could affect the functionality of an addon, so can you be more specific as to what addon is being affected? For that matter, are you sure it's that function that's creating the problems?

Based on what you're asking, auto-self-cast is the best solution and you don't even need a macro. If you're targeting a friendly player, you'll cast Rejuv on them, but if you're targeting nothing or targeting an enemy, it'll cast Rejuv on yourself.