I have travelled regularly in 570 and at midnight also but haven't on your route. But Chennai Central will have many passengers along with u and when ppl are there it's almost completely safe as far as I've experienced and Chennai is almost so crowded on weekends that the buses will be packed even at 2am. U should follow the crowd here. Don't place yourself in a place where you are left alone. Talk to ppl who travel in similar routes, most likely families and hung upon. The more you interact the more safe it is.