the smaller outfits can downsize an HO set or upscale an N one, but I have done this and found that it costs. essentially, for each vehicle you want to decal, you pay about $US130.
for that you will get a minimum run of 20 or 25 sets of the same thing. but if you want a small varied roster, with one boxcar for this road, one hopper for another, etc, rather than a unit train, it quickly becomes impractical.
there are certain microscale sets that include heralds and lettering in different sizes, but for many RRs, including the SCL which is my main focus, the heralds seem to have been a standard size, so you don't get that serendipity where a small HO herald, for instance, can measure up OK for a large TT one.
cheers, ben