Softimage :: Modeling In Symmetry Limitations?

My current workflow is to Clone my object, mirror the polygons on the clone and delete the original half of the clone so I have two working halves. Turn symmetry on so the center points stay on the plane. Then I model - edit and extrude and so on, the original, the clone is updated. Well partially.

Here are the problems I am running into:

1) There seems to be no way to add a primitive into a polymesh. Been through the manual and I did not find a way to do this.

2) This leads to the next problem. There does not seem to be a way to properly merge or join objects into the poly mesh without creating a new mesh object. This in effect cuts the relationship to the clone. And anytime I want to add a primitive into the mesh as a starting point or target for creating new geometry, this means doing the clone, mirror, delete process all over. Not exactly efficient. As note here I am talking about connected parts not just putting parts in that could be separate. Such as contours on a car body that merge to a disc where the headlight would be. Just as an example. And I am polygon modeling here.

I am aware of lofting just so you know, but the above was just an example. I'd like to know specifically about working in symmetry.

Any tips on a better workflow to model - create/add new mesh components including primitives and edit them - in symmetry.