This is pulled directly from his FAQ:
4. If you like my mod and wish to use it in your project I don't see why not. Just credit me, link my mod as a source and if you'll modify those icons - state that you did. I don't want to take credit for what you did with them, especially if you'll ruin my work
Also, would be nice to let me know via PM, or in this thread that you're using them.
Another important point he makes in his FAQ:
3. I don't plan to make unit cards for any mod until I finish modding vanilla.
I think DeI should import these unit cards then snap him up after he has finished modding vanilla units to make the rest of the additional DeI unique units.
If DeI don't want to wait until he finishes all the vanilla units, then I suggest they advertise for a position for someone to make
his style of unit cards for the DeI unique units.
I'm actually a little surprised DeI hasn't leaped on his work already!