The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
Hey guys. I found the solution. I only had to force the game to start windowed.
In steam rightclick on supreme commander FA --> Properties and write in Startoption :
/windowed 1920 1080
...for HD Resolution. in my case it was:
/windowed 5120 1440