You'll have to decide if you want to try it, but I routinely feed Canson Rag Photographique and Aquarelle Rag 240 through the rear tray with no issues. Both are 0.47mm. (Note: I have never fed 4x6 paper.)
Last night, using the media configuration tool, I had to feed Aquarelle Rag 310 in the rear tray for the color calibration step (manual feed for this particular step was not an option). It was difficult to get this 0.6mm paper to feed, but that was a one time action.
BTW, the reason I use the rear tray for the first two papers I mentioned is that I picked a base paper that allowed the rear tray. I later found some doc online that suggests I should have used a different base. I may redo that later, but the profiles are quite good—so I'm in no hurry.