Question is: can trooper choise not to ARO (If he is not CAMO marker and he met ARO requirements) to not trigger perimeter weapons? Does in this situation troopers ARO become Idle and he still trigers perimeter weapons or not?
You can choose not to activate in ARO, but lose it for the rest of the order. Not activating will not cause a boost by perimeter weapons.
They are not mandatory, but if you choose not to ARO against the first skill, you lose any AROs against the second skill as well. https://infinitythewiki.com/Trooper_Activation#Order_Expenditure_Sequence 2. Declaration of AROs: The Reactive Player checks Lines of Fire to the Active Trooper, and declares AROs. Troopers are not forced to declare the AROs, but if a Trooper can declare an ARO and fails to do so, the chance to declare an ARO is lost.