I'm new here but am delighted to find early on in my trawl through various threads that Pete is planning on producing some Bulleid coaches. Fantastic stuff!
Reading through the posts I would back Steph's call to think about the coaches as sets rather than individual vehicles. The Southern were very unlikely other railways and certainly Hornby were caught out big time when they produced their Maunsell coaches. They produced a brake third, composite, first and third. Not surprisingly they had lots of first sitting on shelves, the brake thirds sold out quickly and folk started asking where the brake composite was...it eventually came out, along with a open third and sold well (judging from the difficulty getting them).
I'm sure Pete has been talking to Mike King and he'll provide some sound advice. From my own thoughts, and based on the needs of the smaller Southern layout, my list would be:
1. Diag 2406 Brake Composite. A self contained train ideal for those wanting a single coach for a small layout. Being a 'loose' coach they were added to strengthen other sets (Bulleid or Maunsell) or were used in multiportion trains for some destinations. The ACE used a number.
2. Two coach sets 63-75 comprising of a Diag 2405 Brake Composite and a Diag 2123 Semi-open Brake Third.
3. Diag 2318 Corridor Composite. Can be added inside two Diag 2123 SOBTs to produce a three coach train or used to strengthen longer sets.
4. Diag 2019 Corridor Third. Can be added singularly to the above three coach set to make it four strong or, more typically, two added to make a give coach set SOBT TK CK TK SOBT. Positioning the roof vents differently and installing a different interior would also give you a Diag 2017 Open Third.
So that's five different diagrams, three of which are brakes, but between them you maximise the coach set possibilities. Obviously I've glossed over the small(!) issue of ventilator depths...