Please rethink this. Currently, we can do THF/SWF + SM/ISM which are both combat styles that compliment each other. Allowing Natural Fighting to the list shouldn't cause game imbalance for those who wish to spend the 5! feats to acquire such a bonus.
Second, please break Natural Fighting up into Natural Fighting (level 1), Improved Natural Fighting (level 3 or 6), and Greater Natural Fighting (Level 9 or 12) like the other combat styles.