If it was me I wouldn't bother. Southern's first class accommodation is a very small number of seats at the end of the carriage. Whenever I have seen them I have wondered why anyone would want to travel first class. You get a little bit more room and an antimacassar and that's about it. I've never travelled at rush hour but can imagine you would have difficulty getting a first class seat from East Croydon on any incoming service at those times as I have seen how crowded they are coming through Clapham Junction. Unless you need a seat I would grin and bear it, travel second class and ensure I got one of the fast trains that cover those routes then it's less than twenty mnutes' standing. And there's always a chance you might get a second class seat.