I don't think you'd need an antibiotic. I'm still taking one (somewhat irregularly) in addition to Diane. Bascially I think it is still useful for me because I have these really persistant cysts in my cheeks that kind of fill in around certain times of the month. But if you have none of those I don't think you'd need it. Antibiotics are really weird, you know? Like they are a strange thing to prescribe for acne because they are totally just a bandaid in most cases. I'm trying to wean off them and rely on Diane alone since I know my acne's hormonal. If anything I'd go for a topical antibiotic, less invasive and probably more useful in the long run..
How red is your PIH? Mine was ridiculous and is now just pink (but still super obvious , ugh). Anyway, I really love acne.org AHA because it didn't make me breakout and it's gentle and basically replaces my nighttime moisturiser... I do think it is helping with marks and it makes everything nice and smooth.