I'm going to start with a rationalisation of the motive power for Blackstone. It's been done before, but I'd like to do it again, just for good measure.
As part of the justification for some locomotive classes, part of the background for Blackstone is that the re-laying of track in Bo Peep in the 1900s was done in conjunction with the lowering of the track bed and some locomotives being fitted with lowered fittings such as chimneys and domes. Not every locomotive class would be working the Hastings Direct or East Coastway route, either; some would be working Ashford - Blackstone (corresponding to Ashford - Hastings, roughly).
Some locomotives are listed for fun, or as part of a fictional history developed from the reality. An example of this would be the ex-Tilsbury Baltics being adopted by the SE&CR for boat and other express trains. In reality, the class were too inflexible in route availability and very heavy, causing concern for stability at speed.
LBSCR
For starters, I'm unsure if I need this many locomotives for the LB&SCR to begin with. Yes, it serves Blackstone, but the town's main company is the SE&CR. I could whittle it down to maybe two E4s and an E5 on the Radial front?
The K I'm indifferent toward. I like them, but I can live without it. The J1 I did begin building but I wasn't happy with my efforts - I may strip it down and start again. The I4 was something different, and apparently they were used on Hastings services, which was my initial justification, but I can do without it if it would be a challenge to build. The G class was an oddball derived from Stephenson's survival on the Sunday Eastbourne Pullman until 1914. I supposed I may have a similar surviving locomotive working a Pullman car or two to Eastbourne for a Blackstone portion of the train, but it's a bit of a stretch. I can happily scrap that.
SE&CR
I think I'm pretty happy with this selection. Perhaps scrap the K and the T, though. They're a bit redundant - and the K too short-lived. Allegedly, the P 323 never ran in lake after all, it was historical conjecture. I rather like the idea, and there are other companies which ran their push/pull locomotives in passenger livery so, we'll see.
LSWR
Again, pretty happy with these. I spoke with Linny about the X2. It was just to be something different, perhaps transferred as an experiment alongside the T9s, to see how the class would fare on a hilly, windy route such as the Hastings line as opposed to its usual Salisbury stomping ground. The locomotive is shorter in length than a Schools, around the same width over the cylinders, with the steps a little wider (not necessarily an issue), and all that may be required is a shorter chimney and dome. If anyone finds an obvious flaw with that thinking though, please let me know and I'll amend it.
SR
I don't know that I'll need or can justify two N15s, but I rather like the class. There aren't many early-Southern period locomotives I can sensibly put on the roster, anyway, so this is my lot.
I think that's around 8 locomotives to drop from the list, which helps enormously. There are likely more I can oust, too, but we'll see. I'll do coaches shortly (which will be difficult - I am, after all, a big fan of the carriage).