Pack-N-Plays are available upon request (assuming they have some left). I think you can pre-request them so that it will be waiting in your cabin once you board.
As for fitting in the cabin, it's obviously going to take up some of your limited floor space, but the cabins are big enought that that shouldn't be a big issue since you can fold it up and place it infront of one of the night stands when your not using it.
Since you specifically asked about Cat 8, let me remind everyone that the cabins for Cat 12 to Cat 5 are all really the same size. The only real differences is that in a Deluxe cabin (Cat 10 to Cat 5) has the split bathrooms (does increase your bathroom space) making the 'hall' between the bathroom and closet longer (providing a place for more storage). Cabins with a verandah (Cat 7 to 5) are the same size as other deluxe cabins because the size of the verandah is included in the quoted size of the room. Even the Cat 4 is the same size as the other deluxe cabins except for the extra space provided for the murphy bed (and additional storage on the opposite wall from the murphy bed).