hey ulrich, I think the reason why the person doesn't look like its fitting in the scene is because of the strong contrast between your environment and the colored soldier. It kinda looks like you ran a "negative" filter or some such on the background (or just fudged with the curves), this creates highlights where the shadows should be, etc.
A shadow cast by the character will totally ground him to the scene.
As for the question about the cross and beams. I like the cross, but I think it could benefit from having a highlight, or have scorch marks around the base of it or something (something that will help make it stand out a bit more from the background.)
As for the beams I'm not sure they really fit properly. I would suggest mabye shortening them up some, and only have the "tips or ends" showing like as if the beams have been blown away with the rest of the wall, but you can still see the framing. Here's a few samples from a quick google search for reference
Gallica 82 H
http://www.worldwaronecolorphotos.co...vl00299_p3.jpg
http://upload.wikimedia.org/wikipedi...d_building.jpg
http://www.worldwaronecolorphotos.co.../db_82-611.jpg
and maybe take a look at the wall edges for rubble reference as well...
Anyways hope this helps ya out some
agentxi