Hi Arno,
Nice image! I’m struggling to reproduce your scaling issue, but I think I’ve got it working. Could you send me a file to check?
An issue for assigning materials seems to be that though the family contains (in my example) 2 cubes, the family is treating them as 1 solid 
Here’s a graph using Orchid nodes that associates the family to 1 material parameter, as indicated above Revit Dynamo Combining Spring node family instance by geometry with Associate Family parameter
It could be possible to background open the family once created and add geometry to it? I’ve not investigated that though.
I’ll be going to the BILT hackathon (BILT is too close to my wife’s birthday!) it would be great to meet up 
Cheers,
Mark