They seem to be doing this for some routes, same is on LIRR, but there it is clear what the different "liveries" are I think it is a nice idea, that you get to chose yourself which consist you want to drive (rather than needing to find a service that is run by the consist you want). Note though that not all services are compatible, on SEH I believe the 4-car and 8-car formation share some runs, but also have unique ones each.
So in the games loco list does the 375 and 377/4 count a separate trains or are they lump together as one?
The 375 and 377 are different trains. They are similar in design as they are part of the electrostar family but they run on different routes (375 SEHS and 377 ECW).