I am building a model of a process with Tank, pipelines between tanks, and valves on the pipelines (pretty standard chemical process industry stuff).  I am  running into some fundamental problems in how to handle this in SW (I have 2011).

My general top down approach has been this:

1. Use a series of 2D sketches to define tank position and pipebridge positions.  I had tried Layout but seemed to run into some problems with mates and moving things around with the degrees of freedom I left in.  Anyway, 2D sketches seem to be working.
2. Tanks are sub-assemblies and are generarlly constrained to a floor plan (2D sketch).
3. Pipelines are 3D sketches, one for each fluid or major line.  Pipelines are generally drawn from tank to tank with sections constrained to pipe bridges (reference plane).  Currently pipelines are just sketch lines.  I am hoping I will be able to turn them into proper pipes after I have the layout settled.
4. Valves are Parts (some assemblies), also in this category are flow meters, pressure gauges, etc.    They are generally constrainted to the lines ("pipes") in the 3D Sketches.

The problem is that the Feature Manager Tree doesn't let me fit my 3D Sketch pipelines between the tanks and the valves.  All parts and assemblies have to be together in the Feature Manager Tree, with my 3D sketches either above or below.

This is causing me some practical problems:

1. I am adding the pipelines (3D sketches) after I have  the tanks.  In addition to being a natural work flow, it is easier to draw the sketch and constrain it properly if the tank exists when drawing the pipeline.  That drives the pipelines (3D sketches) to the bottom of the Feature Manager Tree.
2. Copy with Mates doesn't work with the valves (Parts) if the pipelines (3D Sketches) are below the valves in the Feature Manager.
3. I suspect there are other mating problems and rebuild problems that I am running into by not being able to organize my model as described above.

I know this is kind of open ended, but what am I doing wrong?  Or, why can't sketches be intermixed with parts/assemblies in the Feature Manager Tree?

Included is my Feature Manager Tree for reference.

Thanks,

Marty