I have a couple enlarging paper questions and I was hoping to tap into the vast base of experince that I've been learning so much from over the last months.

I've been only getting seriously into photography in the last 10 months or so during which I cleaned up and reopened an old darkroom in the basement of the building in which I work. I've been shooting mostly Ilford delta 100 and 400 and developing in Microphen or Rodinal. I print on Ilford MGRC and MGFB with Ilford's multigrade developer and Rapid Fix then sometimes tone in Selenium. Being inexperienced I've been trying to stick with a limited pallet of materials. I recently branched out experimenting with other papers and two or three questions have come up.

1. The other day I attempted to print on Polymax IIRC in the above chemicals and the midtones came out seriouly mottled in freshly mixed developer and rapid fix. I ruled out that I had mistakenly mixed chemicals or hadn't cleaned trays properly, because I successfully printed on Ilford MGRC and FB papers for the rest of the night. Any Idea what happened with the Polymax?

2. Last night, after successfully printing a negative to MGRC, I decided to have some fun and print to MGRC warmtone for the first time. Using the same exposure, f-stop and contrast filter for the warmtone I came up with an underexposed print, about two stops/zones short. I was able to get the print I was looking for by opening the aperture one stop and doubling the exposure (a little less than double to be exact). I couldn't find anything in the pamphlet in the pack of paper to say that there was ISO change between the papers. What happened?

3. Does anyone find MGFB in a half or full stop slower than RC? They're both rated at ISO 200 with filters #0-3 and 100 with #4-5, right?