You can only play with 31 civs in any one game - there is a hardcoded limit of 32, and the Barbarians always count as one.
You can customise the mapsize to even bigger than that. Not sure what the limit is, but I will check.
EDIT: In the editor you can make a map size 362x362 as the max, but I suggest you stick to smaller, as it is taking forever to make that map on my computer, and sucking all the system resources.
The corruption is more a factor than in PtW as the FP has been toned down. This was changed so it had more of an effect like it does in PtW for the first beat patch, and (reportedly) has been made even more effective at combatting corruption in the NEW beta (I have yet to test this).
Flip answered the last one perfectly.