Regarding the Hollowcloak, it is supposed to be rather niché, and not fit in a regular army. If it gets caught with its cloak down, so to speak, it will be mauled. The AI probably won't use it very well, which is a slight problem, but I still think we deserve some fun toys. I also think the game mechanics work so that no unit may ever end up with negative strength: 2 Str, -125% would give something around 0.7 Str - next to nothing, but still not actually negative.
Regarding the Skirmisher, I was going to do some research on its Withdrawal chance with innate ability and Flanking promotions. I feel it should wind up somewhere in the neighbourhood of 90% Withdrawal, maybe even 98%, but have only 1 movement. The Skirmishers' weakness should be retaliation and its inability to attack cities, in my opinion. I think the current design accomplishes that rather well, without having studied the details.