Don't own or have experience with the EPA 250 but I would expect the following:

1) Stated effective mass of 14 would include the headshell.
2) The weight of the Art 9 is perfect for the EPA 250 (falls in the weight range where neither headshell weights need to be used because the cartridge is too light or a heavier counterweight needs to be used because the cartridge is too heavy).
3) Compliance on the Art 9? This is a tougher one but based on the recommended tracking force and the fact that it's a MC cartridge I think real world compliance is probably going to be in the 15-18 range maximum. Highly compliant cartridges most often have lower tracking forces and you also don't see MC's in the real world too often that are actually highly compliant.

In short, I would think the ART 9 would be a very good match for your new arm. There's a pretty lengthy thread at Audiogon with multiple users of the cartridge being very happy using the cartridge with different arms in the 12-16 gram effective mass range.

The ART 9 is supposed to be fabulous. Go for it!