It's easy to use the fitting room to a non-zeroed figure as an interim step. But the final rigging has to be to the zeroed figure, unless you want to have to pose the resulting cr2 every time to match. You simply have to leave a thigh gap, and I recommend doing all the adjusting with the morph brush. Maybe you can reduce the thigh of the garment just enough along the X axis, and if necessary morph brush the inner thigh back out after rigging the garment. I've done that. I almost never run the fit sim in the fitting room. Also, it is best to rig to the base version of the figure and then copy the character morph from your figure.