Steppers as their name implies have steps, you can't really accurately move less than one step. Some controllers have "microsteps" but they are more about smooth motion than actual steps.
So about the minimum you can move is ~1/200th of a turn, there ought to be nothing to stop you doing that very infrequently, but the 1 inch / minute might be a limitation of the controller software.
As far as I can see EMC has no issues trying to move the axis at 0.02 inches a minute, I haven't tried it on my mill.
It's also possible that the steppers don't have enough torque or might stall at that speed, but I have no expertise here.