No, it's not an arbitrary rule. As I said, the show has established that if an adult who isn't about to die has a Traveler or Messenger downloaded into their brain, it kills them within moments. We saw that happen multiple times throughout the season, like when the team was about to diverge from the Director's plan and a Messenger was downloaded into an adult to warn them to stop, causing that woman to drop dead a moment later. (Which is also what happened in the Helios example you mentioned.) Or when the Faction was attacking the team in the farmhouse and they blew up the jamming device so that the Director could download into the attackers and kill them all. We also saw it in the episode where Grace/0027 was put on trial. In the climax, the Director spoke to her directly by sequentially downloading itself into multiple terminal hospital patients who died within moments. (Presumably they weren't about to die imminently, or else it would've worked like a normal Traveler download. They may have still had a few days or weeks.)
So this isn't a matter of policy. It's one of the physical rules of the process, that a download will kill an adult unless they were going to die anyway. Thus, it shouldn't be physically possible to change that.
Perhaps the difference is that the Faction consciousnesses were downloaded into the quantum frame in 2017 first, and then uploaded into people's brains. Maybe without the time travel component involved, the transfer isn't fatal. Which could also explain what Ingram did in the finale.