Can't now see the Old Firm joining the Premiership, maybe a few years back when all the top names were up in Scotland they may of had a chance of mid table in the Premiership, but not anymore. They would struggle for the first few years until they got the money in and could pay for the top names again, then they could be competitive. But with the present teams they would struggle for yop 6 in the Championship IMHO.
Plus they would have to turn their back on European Football, having been guaranteed Champions League and UEFA Cup Football in Scotland.