I have been fiddling recently with a very similar project using MCL2P's and passive speakers ...both in-wall and wall mount in a room about the same size.
It is very much a work in progress and may actually be moved yet again! 
I only have the centers and fronts hooked up currently and will probably not go any further until I decide on its final resting place.
Currently...
BS1 > BV4
Center: (1) PL cable to an MCL2P / (2) leads out to a pair of Anthony Gallo Nucleus speakers (in-wall socket mount).
Fronts: (1) PL cable to an MCL2P / (2) leads out to a pair of BeoVox 3000 passive panels (wall mounted).
Because all the speakers are passive, they seem to have a common warmth to them. I had to balance the distances in the menu to accomodate for the sizing of the speakers. The result has been very good.
I had considered moving a pair of BL4000 or BL3000 (problem is the displays are bothersome for TV viewing -I think they are better for a link room) into the front position as the actives are just so brilliantly "clear". I would then find myself in a situation more like you -a mix of passive and actives. Personally, I think that it might make more sense to have the actives in the center... providing nice crystal clear dialogue and then have passives as the fronts where you can pick up some bass. With 6000's that probably isn't an option for you. In your case, I might even considering looking for a single or pair of BL4000 for the center??? and using the passives for the rear.
Regardless, I think balancing the warmth of the passives w/ the clinical clarity of the active speakers might be a challenge. I am sure someone has a similar setup w/ mixed speakers here!
If you move forward w/ your plans as they currently are... I'd suggest concentrating on the centers for now. Before cutting into the wall, set them up on a box or something at their finish placement level/position and play w/ the balance of the speakers. Spend some time listening. If you are happy w/ the results... keep in mind that once in the wall -the sound will improve. Then move on to the rears.
Question: Does the Beosystem 1 output both left and right channel on
both "rear sockets" (so that I only can use one powerlink cable to the
Beolink amplifier)?
Yes. As above, I have single PL leads from the audio outputs on the BS1.