Here are some notes that might be helpful to keep in mind.
1) Switch rod and Spey rod rating are typically different from the single hand rod rating. For example, the power level of a 5wt single hand rod is not the same as the power level of a 5wt Switch rod. Neither is the power level of the 5 wt switch rod the same as the power level of the 5wt Spey rod. You will need a heavier line to load a 5wt Switch rod compared to a 5wt single hand rod. Similarly you will need a heavier line to load a 5wt Spey rod compared to a 5 wt Switch rod.
2) The grain window listed for the Switch and Spey rods are typically rated for Spey type casting (either Scandi or Skagit type casting which forms a D-loop) and not for overhead casting. Scandi type spey casting will typical make use of the lower end of the grain window whereas Skagit casting will make use of the higher end of the grain window. For example TFO Deer Creek 5wt Switch rod has a grain window of 250 - 450 grain. If you plan to do Scandi casting with the rod, choose Scandi line with grain weight around 250 - 300. If you plan to do Skagit, choose Skagit line with grain weight between 350 - 450.
3) Not all rod makers list the grain window of their Switch or Spey rods. If the grain window is not listed, a convenient way to find the suitable grain weight is to refer to the RIO website for Spey line recommendation ( http://www.rioproducts.com/learn/spey-line-recs ).
4) If you plan to do overhead casting with Switch rod using line designed for single hand rods, the grain window does not apply. In this case, a good guideline will be to over line the rod by 3 to 4 line weight. For example, if you plan to overhead cast a TFO Deer Creek 5wt Switch rod, do not use a 5wt line, use a 8 or 9 wt line on the rod instead.