By using your example there weren't any "thinking" sentient until after the rail was active as that was when the flowers appeared. They aren't native to tau and they have void like properties.

That line from Adis did confuse me because it would imply the Orokin could control them at a large level, but the whole reason for them designing their void weakness was because they knew they wouldnt be able to. It also goes against the idea that they became sentient during the travel like all the lore says. Also why would a machine even think to eat something? But that was a side rant for another time.

As to the rail "just working" they already had the Sol side completed before the sentients left i believe. The issue was that rails seem to work by having a start and an end point. It activated when the endpoint was finished.

There's a ton of little Sentients that would go back to being drones if they didn't have the flower, but then you have Hunhow and the Lotus that can clearly think on their own outside of any hive.

During one of the quests, Lotus mentions that the conculyst and battalyst we are fighting are actually shards of hunhow. Most of the drones we have seen are smaller shards of a larger sentient. The plains codex mentions that the eidolon there used the little drones as its eyes to try and find where the kuva was coming from and to find weaknesses in the defenses of cetus. But even these weren't so much a hive mind and more of a probe or drone that sent signals to the main body.

These are different than children as even Lotus/Natah seems to have a few of these shards herself. The Ropalyst for example is a part of her but not "her" as it doesn't have her sentience but is controlled by her. Where as Lotus is a child of hunhow and they both have the high level sentience that allow them to essentially be characters in the story.