Hmmm, Kay is onto something here. I wouldn't say 12 posts per person, but perhaps maybe a week max per round (maybe even five?) before someone decides the winner? I say this, because if during a match two people are active and on all day, don't limit them to 12 posts. Reward them for their activity, let them go at it! Though I do see the point in 12 posts to speed things up, which i why I recommend after a week, if a match is not finished, then a winner would be announced (This would mean with 4 rounds, it would take a month maximum).
As for restrictions, I'm not thinking any. KG, implants, all are fair game. Use whatever powers you have available to win.
As in regards to my opinion on voiding, here is my opinion. They should be allowed to void something if it is unreasonable. However I would further propose this rule. If you have multiple posts invalid, you get dq'd. Basically if you are getting constantly told to repost (perhaps after the second or third time) you are not skilled enough in formatting your post and should not hold a tailed beast, thus are dq'd. This would point people to not being so ridiculous with their posts. Perhaps have a similar deal if someone is being a crybaby and trying to void posts that are legit? Just food for though.