Base training in the A321 won't happen simply because there aren't that many in the fleet compared to the A319/320s plus of course it costs the company slightly more.
The 6 month restriction on A321 landings for FPP cadets only really affects the line training on an A321 sector as the majority of new FPP's are sent to LGW where the A321 doesn't operate from. By the time those that request transfers up to LHR get their moves, they will be relatively experienced and well past the 6 month restriction.
Champ