6 Ohms won't be an issue and mixing impedance isn't a problem either. If they were 4 Ohms, then there might be a some concern depending on the receiver, but likely only the lowest models might have an issue with 6 Ohms when pushed hard.
Passive biamping is pretty much pointless, so no worries there either
It is hard to quantify "a LOT" better, but many of the mid level Polks are not exactly highly thought of around here. The problem is, everyone's preference is different. I consider the FS52 better than the P363 (and I've side by side compared the 163 and the BS22). Have not heard any recent BICs or Fluance, though I'd still tend to say the FS52 will be tough to beat in the price range and the AALs even harder (I have not heard them actually, but I have no doubts about Dennis's work, heard quite a few of his speakers).