Just noticed that you've missed the multiweapon fighting penalties. The attack line should read:

6 Masterwork Scimitars +10/+5 (1d8+3/18-20) or 6 Slams +11 (1d8+3)

The creature is also really weak for a large creature. If you went for a more conventional minimum of strength 20, it would end up as

Attack: Masterwork Scimitar +14 (1d8+5/18-20) or Slam +13 (1d8+5)
Full Attack: 6 Masterwork Scimitars +12/+7 (1d8+5/18-20) or 6 Slams +13 (1d8+5)