Yes, you can use the Hornby coaches in rakes that mix K-type (current) and all-steel K-type (to be released) cars. The sections towards the back of the two Ford volumes you refer to Jamie should tell you what you need to know. Then you will need to read the detailed chapters carefully to establish whether you need K-types with matchboard or plated over sides, original or later Pullman crests, names, numbers, 3rd class, 2nd class, no class, moved from one region to another, rebuilt from parlour to brake ...
You may find it takes longer than you expect!