For every adventurer, I travel all the way to my retired fortress, and select "Rest at this location (...)". I then go to fortress mode, check out two or three things, kill the dwarfFortress.exe process (so no saves of the unretired fort are ever done), and create another adventurer.
-> Race : Dwarf. Civ : My civ. I "created" him on my fortress square (just cycle through peasant and locally important) for the sake of efficiency.
=> It works ! I can control him perfectly.
=> So far, I've seen nothing I can't do that a normal dwarf can.
For later use, this "perfect result of insertion" will be called "result OK".
-> Race : Dwarf. Civ : Human civ.
=> Does not work : "No labors available"
=> You CANNOT cheat on dwarf therapist and get him to work either, so he is basically useless.
=> He can be made part of your localisations (inns, temples, taverns).
=> You cannot get him in your military. Cannot get him to be noble.
=> Your squads cannot attack him.
=> Burrows work with him just fine.
=> You can assign items to him (bed, chairs, tables, ...)
=> He has the labors : "Cleaning", "Constructions", "Haulings", "Nurses" and "Lever". These job can't be removed from him.
For later use, this result will be called "result KO". The creature is kinda like a grown up child.
Even if the "result KO" creature has some experience in any skill (say, butcher), this skill will be disabled in fortress mode, even if said skill is the highest one.
-> Race : Elf. Civ : Elven civ.
=> Elf got killed as soon as I embarked : found dead dehydrated.
=> So I save scummed a little, made my adventurer drink, retired him again...
=> Result was "result KO".
From now on, I won't create adventurers from new civs no more, I guess they are never controlable.
I will aventually make some research on what happens if an adventurer is a hero in your civilisation's eyes. Is he then playable ?
-> Race : Goblin. Civ : My civ. Not my fortress square.
=> Result was "result KO".
-> Race : Goblin. Civ : My civ. Created on my fortress square.
=> This time, result was "result OK" !
So, adventurers "created" on your embark tile will be under your control. Completely.
Other will have the same traits as the "result KO" dwarf above.
I made sure of this fact by creating an human from my civ, starting on my embark square. He was result OK too. So it seems to proove that the starting point of your adventurer make all the difference.
-> Human outsider. Not my embark square.
=> Result was "result KO".
-> Human outsider. Created on my embark square.
=> Result was "result KO".
That one was a little sad, but I guess you have to be from The Still Volcano civ. to be "result OK". Outsiders have no civ.
-> Race : Elephant-man. Civ : My civ.
=> Elephant man is "result OK"
The Elephant-man is wearing "large" clothes. I made him a leatherworker and ordered some clothes made. He crafted these as "large" too. So it seems that the creatures craft things based on their own size.
-> Race : Eagle-man. Civ : My civ.
=> Eagle man is "result OK".
Sadly, I haven't found any evidence that the Eagle-man can fly in fortress mode, yet. I tried setting some stockpiles in the air. The Eagle-man doesn't fill them. Tried attacking some ravens too. Surelly the path-finding algorithm doesn't deal with flight.