I am able to copy the iCal icon while using PathFinder Get Info and copying it onto a generic Folder icon in both PF and Finder Get Info windows/dialogs. I'll see how it goes the other way around, I'll pick a non-Apple icon, also.
Calendar icon copied from PathFinder Get Info dialog
Monosnap icon copied from Finder Get Info dialog
Works in Finder to Finder Get Info windows, also. This time I used the icon for MonoSnap, a screen capture app. These icons are fully enlargeable.
I don't know what the OS version of the icon/images might be, I assume they are .png. Actually, the icons are ".icns".
1,024 x 1,024 pixels. I think they moved to that size in Mountain Lion from the 256 x 256 in earlier OSs.
Only real difference I see is how the app shows the selection. Finder's is slightly better, I think:
PathFinder - - - - - - - - - - - - - Finder
Is there a particular icon that's giving you trouble? If you really want it, you can usually find the ".icns" file in /Applications/the App icon you want directory.