When natural attacks are present with power attack, the only option given to apply a temporary power attack bonus to natural weapons is the two-handed-power-attack. This is incorrect. Primary Natural attacks should get the one-handed (1x) power attack, and secondary natural weapons should get the off-hand (.5) power attack.
There are some natural weapons which apply the 1.5x power attack rule, but those are specifically stated, and an exception rather than the general rule. Specifically, creatures with only a single primary natural attack add 1.5x Str bonus to attacks (and power attacks, consequently.) There may be other exceptions.
"Most creatures possess one or more natural attacks (attacks made without a weapon). These attacks fall into one of two categories, primary and secondary attacks. Primary attacks are made using the creature’s full base attack bonus and add the creature’s full Strength bonus on damage rolls. Secondary attacks are made using the creature’s base attack bonus –5 and add only 1/2 the creature’s Strength bonus on damage rolls. If a creature has only one natural attack, it is always made using the creature’s full base attack bonus and adds 1-1/2 times the creature’s Strength bonus on damage rolls. This increase does not apply if the creature has multiple attacks but only takes one. If a creature has only one type of attack, but has multiple attacks per round, that attack is treated as a primary attack, regardless of its type."
In the Universal Monster Rules for Natural Attacks.