[J0sh...]

PHP код:

public OnPlayerRequestSpawn(playerid)
{
    if(GetPlayerSkin(playerid) == 26)
    {
        new SkinName[MAX_PLAYER_NAME];
        GetPlayerName(playerid, SkinName, sizeof(SkinName));
        if(!strcmp(SkinName, "Apex", false))
        {    
            SendClientMessage(playerid,red,"This Skin For Apex Only");
            return 0;
        }
    }
    return 1;
} 


Not sure if it works.

EDIT: Didn't see you solved it, good to here.
OnPlayerRequestClass is when the player switches to a new classid.
OnPlayerRequestSpawn is when the player clicks the spawn button.