Answer to questions - "yes!", but, most of us don't have up to date numbers regarding actual attrition/retirements. Expanded explanations, regarding your questions are on the other Southern thread under "Cargo".
Southern Air
My actual reply, is to ask - what is your real question? SAI, is not a legacy carrier. We have a wide range of ages, in all seat/airframes. At present, Seniority is not a requirement for upgrade on the Classic; so, ....? If you're wanting to know if you'll be a Captain on the B777/B744 within two months of being hired - well, you'll just have to apply and see!
If you have a more direct question; read the posts on the other thread and ask away.
Biggie