Maybe I'm just tinfoiling again, but what about the Oresrians?
It would fit almost perfectly in every encounter we had until now. The so called "Thargoids", are Osresrians: That's because the Unknown Ship just scanned us, that's why they ignore us completely on the Barnacles Sites but defended themselves when they got attacked by the Federal Units in Maia.
The more peaceful Osresrians look the same way as Thargoids do and have only minor differences.
(Unfortunately,it takes a highly trained eye to tell an Oresrian apart from aThargoid. The standard advice is to assess the thorax markings, andlook closely at the shape of the hind leg's fourth joint, which inOresrian anatomy should be noticeably more convex. http://wiki.alioth.net/index.php/Oresrians)
The Wiki Article also mentions, that it is Important to see the whole Situation in an encounter. If they are attacked they will be shooting back of course, and if someone is accidentally mistaken the two races, they also react pretty aggressive.
That's in mind: We have more signs, that they are Oresrians than Thargoids, a Thargoid probably killed us in under a second.
Just a theory but I thought I post it, just to make sure it's written down somewhere