Some interesting combinations must have been possible with Southern Region slam door EMUs. 12 car formations consisting of 4CIG+4VEP+4CEP? Or (less likely) an 11 car formation consisting of 4CEP+4VEP+2HAP+MLV? Don’t know if either actually happened, and if they did happen if they were commonplace or not, just occurred to me that they would have been possible.
CEP+CIG+VEP was most certainly possible, on both the South Western and South Eastern sides. I’ve no idea how common on the SW, however it was comparatively rare on the SE simply because they only had a small CIG fleet, so law of averages meant it didn’t happen often.
At various points in the 1990s and early 2000s WAGN had a fairly varied fleet. Apart from the 365s, at times there was a certain amount of variety within their 317 fleet. The 24x 317/2s were refurbished to 317/6 with a 365-replica interior. On top of that the 317/1 fleet varied between NSE, LTS and WAGN purple livery (albeit the interiors were all essentially the same). Then around that era the Stansted units very occasionally made an appearance elsewhere, and the 322s had a very brief spell appearing on the GN side too. I remember in the 1990s there was a morning Letchworth to King’s Cross service which was booked for 2x313, but in practice could turn out 1x365, 2x365, 1x317 or 2x317, with scope for variety in what type(s) of 317 would appear.
The WAGN/One split generally put and end to all this, however the One franchise then managed to create four distinct versions of 317 interior, which in practice often strayed from their intended service groups.