You can add another variable "id" and add this;
example//
new id, skin;
if(sscanf(params, "ui", id, skin)) return SendClientMessage(playerid, -1, "usage: /skin <Name/PlayerID> <SkinID>.");
And maybe this to prevent anyone from using this
if (!IsPlayerAdmin(playerid)) return SendClientMessage(playerid, -1, "You are not admin!");