Depends how the paint responds to the PP. In many cases, the PP will leave a ready-to-wax finish, in which case I would not use *either* the P21S GEPC or the AIO, just wax. The waxes in PP aren't something you need to remove, IMO.
In other cases, the PP leaves *very* fine micromarring (less likely if you work it a while). In that case, use the GEPC (though I greatly prefer the 1Z MP or WPS).
But I dunno if I'd use AIO in either case. Both the PP and the GEPC are fine bases for the final LSP, without the AIO. I don't know if it would add anything.