Hi there

I would personally upgrade to Poser Pro if you could and this would definitely help you as you already have enough memory to handle,limit of 32bit on Windows is around 3.5GB,not sure what is limit on OSX,but I would think will be around 4GB

Yes Cloth room does work with different figures,I'm using this on several figures like on new or older figures and if Copy morphs does work with non SM figures yes it works,I often using this on clothing as some clothes does comes with few morphs and sometimes you need bit more 

Usually I do render with 3 and more figures and often my scene is large,last scene which I've rendered has have several props(houses) and 4 V4 fully multi clothed,with multiple morphs and hair etc and my scene fully loaded started on 4.5GB and when I rendered RAM count jumped to 9GB RAM,I've got 32GB RAM and when I do render I've opened Chrome with multiple tabs and my RAM sits at 45%(I've got 32GB RAM)

Not sure what is limit of the polys in Poser Pro,but you can find limit fairly easy if you are like to render multiple people and there is fair amount of the props,this is my issue with Poser Pro,but really depends on yours workflow,usually I export some figures as OBJ and then reimport them to Poser and this does helps with lower polycount,if you do only static renders as I'm doing

And I would agree is worth to upgrade to Poser Pro if you are not on Pro version or I would wait on PP12,not sure when PP12 will be released,but hopefully in few months should be here

Hope this helps

Thanks,Jura