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"
]