Disruption during movement.

[Mowgli]

You're right. This is a small difference between the ACW and Napoleon games: In ACW, the strength used for shock tests is always 25, whereas in the Napoleon-games, it's "current manpower/10" (capped at at leat 25). I have to say the ACW method (which is similarily also used for the Panzer Campaigns series) makes more sense to me as it doesn't blur the difference between unit size and morale.

Other than that, ACW seems to work like I have described here, with big and small shock tests and big and small morale tests. You can also judge by the command report: "X units routed out of Y units tested". Y tells you, how many units have failed their big shock test and are now at least disrupted. X tells you how many of those units have failed their big morale test and have routed. (The other ones have passed their big morale test and are thus only disrupted)

You're correct that being successfully assaulted leads to an automatic (big) morale test (the shock test is skipped).
Same is probably true for units suffering casualties when at maximum fatigue, but I haven't tested it. I wonder if max. fatgiue makes a unit skip both big and small shock tests.