For different scale drawings (layouts) I generally have a different drawing. X-ref the "base" drawing into it, and data shortcut the pipes into it. That way I can adjust the labels per drawing.
If you want the lables the same size regardless of the drawing scale you can create child styles with a smaller text size, and have 2 labels per pipe. I have not gone to that extreme, but instead I have tricked the labels into thinking they are the same scale. I scale the border in the layout, and have the vport the same scale as the other plan. I plot so the actual plot is the right scale.
Maybe in more modern versions you can set the annotation scale for the vport? Can someone with a later version respond to that part? (I am using 2008)