Although using ied and npm at the same time for installing dependencies should work, you might run into some weird corner cases.
Therefore, npm and ied are compatible in the sense that you can use ied to install packages and npm for publishing them. At least that's the level of compatibility that is currently supported.
Good to hear, I reckon I could quite happily use ied for all the installing stuff if it's going to produce the same tree as npm, but perhaps not support all of the crazy corner cases.