These particular aliens are still a spoiler and are a special case you cannot have diplomacy with.
...sorting out diplomacy is complicated. Esssentially they do not recognise your (or anyone elses ) claims to a star system
Civ2 had the same problem, IIRC. If I could accept it from the king of strategy games I can accept here. But I would strongly suggest to Steve that he hire all of Firaxis to work for him on Aurora. That would help smooth these things out.
I got lucky and didn't have to decide whether to use the information from the forums about those guys - My contact team just threw up their hands one day, decided that we would never be able to communicate, and the aliens automatically flipped to 'Hostile' somehow.
I'm a bit scared of them now, though, as it took 24 FAC armed with 20cm spinal-mount lasers an unreasonably long time to blow up a single 6000 ton survey ship of theirs.
Of course, less scared after a couple of 12800 ton ships jumped through the wormhole and Home Fleet was waiting for them. Flushed half of the missile racks at them and the Standard-3s vaporized the enemy before the heavyweight missiles even got there.
I'm going to have to find a happy medium somehow.