The whole argument around Thameslink running 12 car DOO is a bit of an accident anyway. WAGN negotiated a 12 car agreement as the was no other way of increasing capacity on the Kings X line. The agreement actually states that as 365s have no walk through between units, all 12 cars would, where possible, carry rpis in the rear unit. This agreement, like that of the OBS on southern, never came to much and if rpis weren't available trains ran single manned.
Upon harmonisation under FCC, Thameslink sort of accidently inherited the DOO 12 car agreement which FCC started to use. The union couldn't do anything as they had, perhaps without realising it or its importance, agreed to allow it at harmonisation.
GTR stated when it took over it would not be harmonising FCC and southern drivers and continues to state that now. However they are trying to harmonise bits by the back door - without negotiating or paying for it. To be clear, southern and gatwick drivers have a completely different employment contract-different pay, conditions and DOO agreements to FCC drivers. So any comparison between FCC and southern drivers and trains is pointless as they remain seperate companies wi seperate contracts and agreements.
Thameslink never formally agreed to extend DOO to 12 cars. It was purely agreed to increase capacity on a few evening peak trains between London and Cambridgeshire.