Adding indepentently resizable symbols into your custom shapes is possible by creating subshapes. This is done by some extra attributes on the group element.
      
        
<svg:g
 subshape="true"
 v_anchor="fixed.bottom"
 h_anchor="fixed.left"
 default_scale="1.0">
 
      
    
"fixed.top"
            "fixed.bottom"
            "proportional"
          ]
        "fixed.left"
            "fixed.right"
            "proportional"
          ]