Are you fishing the rocks or the beach?
Either way, use the lightest sinker you can get away with or none at all. I like to use an Alvey and 4-6kg line. I don't think you need anything heavier. With an Alvey you want a good swivel ABOVE the terminal to minimise line twist, so I either run a ball sinker down onto the gang hooks with a swivel above it, or, if I want the sinker back from the hooks a bit, I have the gang on a short trace, then a swivel, then another short trace for the sinker then a swivel to the main line.
With a thread line or an overhead, you can just run the sinker to the gang, or set back above a trace.
Hope that helps.
Baz