The rules around needing to declare your AROs that you know to already meet a validity requirement after the first short skill read as follows: "The Reactive Player must declare AROs for all eligible Models or Markers immediately after the Active Player declares his Entire Order or the first Short Skill of his Order (see: Order Expenditure Sequence). Troopers that fail to do so lose their ARO against that Order." Notably, it calls out eligible Models or Markers here as needing to ARO immediately. Does this mean that troopers in HD always have the option to wait until after the second short skill of an order before declaring their AROs?
No you cannot delay the aro with them unless the active model is a marker etc. At which point delaying will cancel your hidden status Sent from my SM-G973F using Tapatalk
Seems pretty clearly meant to include Troopers in HD as well, though you're right that such a Trooper is neither a Model nor a Marker. But if we're going for super-RAW, I'd rely on the second half of the same paragraph: "The Reactive Player must declare AROs for all eligible Models or Markers immediately after the Active Player declares his Entire Order or the first Short Skill of his Order (see: Order Expenditure Sequence). Troopers that fail to do so lose their ARO against that Order. If, by declaring the second Short Skill of its Order, the active Trooper gives an ARO to enemy Troopers that did not have ARO against the first Short Skill, then those enemy Troopers can declare their AROs." So, you could argue that the HD Trooper doesn't lose its ARO pursuant to the first half, but the second half only allows Troopers to ARO if they didn't have an ARO against the first Short Skill. So the HD Trooper hasn't "lost" his ARO, but he nonetheless can't use his unlost ARO since the rules would only permit him to use it if he had not had an ARO to the first short skill. Which is obviously pretty convoluted, so, to put it hopefully more simply: the fact that the second half of the paragraph refers to "Troopers," including HD Troopers, shows that the first half of the paragraph was intended to include HD Troopers as well.