[Tim Kauffman]

The ability to only move similar unit types in a stack even if it also contains other unit types and you would then have to go back to pick up and move those individually would be such a big improvement in the game play experience. After that update is implemented, then finish it with a final update by being able to move entire mixed stacks.

Maybe you could have this as a two option feature.
1)Move all similar units as one stack.
2)Move the entire stack of remaining mixed units.

This may prove to be a nice option by having two choices that would allow more intuitive gameplay.