The 36 doesn't need a matching fleet - some 06 Darts and a pair of 53 plate Geminis will be fine.
You're right that the 903 will release some Geminis and with all the other changes but who's to say that they'll be staying?
As an aside, FHD will require a fair few deckers for Brock college and Dorset schools so wonder what they'll get?
But by having a common fleet or vehicles on a service it aids allocation by having to make sure certain vehicles are allocated to specific cards. This would allow when off for maintenance a Dart is put on a card which doesn't require a decker and there isn't a struggle to find a decker for the cards which do. Also, at times of disruption when services are messed up or a service is missing the capacity is there.
Several colleagues catch the 36 into work and all say that if the service they catch is a decker it is fine, but on the occasions when it is a Dart it struggles, particularly from St Anne's and Barton Hill into the centre.
Freeing up Darts may also allow services currently using short E200s, to be converted to Dart operation to add extra capacity. The 3s in Weston are mainly Darts now, but when an E200 turns up it struggles. The short E200s are just too small for most routes, unless it is dictated that a short vehicle be used. The 3s in Bath could probably do with the short E200s being replaced.
Within First now the aim is to have a common fleet where possible on a service, to ease allocation, ensure capacity and promote a consistent brand. It doesn't always happen currently, but it is getting there.
Who knows what will happen with the displaced Geminis, they may be shipped out or may be kept to see of the remain W reg ALXs or some of the ad London B7s which are shorter. In September extra deckers are being put on the X1/X2/X3/X4 circuit so some may find there way to Weston, until spare E400s can be sourced.
