You can install all 3 (should be ok with 64 bit and 32bit mixing)as long as they are on there own partition and you shouldn't lose anything, but it is always good to back up what you want saved just incase something does go wrong.
Not sure about hooking up 2 pc's to 1 monitor, possibly could be done although I would think you would need a switch.
VGA,CRT,DSUB are all the same connection just different names.
