To offwer some potential logic, I always think you need 4 types of locos for a good mix - top link tender, mixed traffic, small tender or large tank and small tank. To apply that to Farish you have the following
Southern - Merchant Navy, N Mogul, C Class, Tank missing
LNER - A1 and A2, B1, J39, Tank missing
LMS - Duchess, Black 5, Fairburn Tank or 4F, Jinty
GWR - Castle, Mixed Traffic Missing, Small Tender/Large Tank missing, 64XX
So there is 4 main gaps to fill - ok there is a lot more to it than that but if you could fill those 4 gaps you would do well for variety