The heroes have no boons, so I deploy the Elite Probe Droid as instructed. My question is, are the HK Assassin Droids available for deployment elsewhere? Seems odd that I would be prevented from using them in this mission.

Reserved Groups are only deployed from mission events, if they happen. There are core campaign missions where reserved groups are never deployed due to options that are not selected.

Short answer: You can't use any reserved groups for normal deployments unless a mission rule specifically allows you to.