An rpc (not rcp, it stands for _R_emote _P_rocedure _C_all!) will be send to all other known peers, whereas rpc_id will be only send to the one owning that peer-id. You can pass an id to rpc, but only as an argument for the called method on the other side. It won’t change to which peers the RPC is send.
Thanks and sorry for my dumb mistake, had a lot of stress today trying to understand networking.