The coordinate system used in the WMS map image is very important. It is what enables the WMS layer to match up with your other layers. Make sure you retrieve WMS Server images that you can use with the other maps you use. Keep in mind that the raster layer always determines the projection of the map. If there is more than one raster layer in a map, the largest raster layer determines the projection of the map.
When choosing the projection for a WMS map that is made up of several layers, the projection choices available to you will be only those that are common to all the layers.
WMS servers may offer data in projections that we do not support. Be sure to choose a supported projection.