Bluemoon gangsters are 18 mm but that is a far cry from 25 mm - unless you mean their 28 mm range which I have no knowledge about. Don't see the range of cars they use to have (in 25/28 mm).
Not familiar with the old Citadel gangsters but they'd have to be Really Small to match to 18 mm figures.
I'm doing it in 18 mm including the buildings offered by Bluemoon - and sourcing vehicles from other ranges, WWI often useful, sometimes Matchbox cars, also Spanish Civil War stuff can sometimes work. Of course, I am not a 'purist' so as long as fits in well enough, that is good enough for me.