Ross, I would suggest your first port of call to be this site - https://groups.yahoo.com/neo/groups/BRCoachingStock/info
It provides the train marshalling arrangements for the ScR in 1948, which will give the basic formations.
The HR mainline was complicated to operate as it had two routes to Inverness, both of which operated through coaches. This led to a mix of ex LMS and LNER stock, for which the LNER had set vehicles with compatible corridor connections. This has been discussed and from memory, I think it is in this thread - http://www.rmweb.co.uk/community/index.php?/topic/65351-the-glenfarg-route/?hl=glenfarg. [see Page 7 which includes the 48 Carriage information] A great deal of shunting and re-marshalling took place in Perth.
There are a couple of things to bear in mind for that era. There was still considerable pre-grouping stock available, which was often used to strengthen the marshalling instructions and the ScR applied the maxim that if it moved, use it. That and between Perth and Inverness there seemed to be a limitless supply of Black 5's to pull whatever was required. My father still tells stories describing monstrous trains coming into Perth from Inverness, with multiple Black 5's. These being replaced with a Stanier Pacific returning south.
I am sure there will be much more information on tap here as required, but this might give you a start.
John