Small creature, occupy and flank?

[TheApapalypse]

Can a small creature move from a threatened square into the enemy square, (Rangers companion, Dire Rat) occupy and still contribute to flanking?

[Firebug]

Count as flanking? Not initially, but there are a few options.
First, how big is the creature whose square it is occupying?

Because, if it's a medium or large size, they can't be in the same square (barring special rules saying they can).

I am just going to assume the enemy is at least Huge. Improved Outflank, Pack Flanking, Tribal Hunter(though requires others to be flanking), Gang Up(though harder to hit that 13 int), and Blades above and below(though one of you would need to be huge...) are a few options.

[Taja the Barbarian]

Generally speaking, you can't share a square with another creature:

Moving Through a Square wrote:

Source PRPG Core Rulebook pg. 193

You can move through an unoccupied square without difficulty in most circumstances. Difficult terrain and a number of spell effects might hamper your movement through open spaces.

Friend: You can move through a square occupied by a friendly character, unless you are charging. When you move through a square occupied by a friendly character, that character doesn’t provide you with cover (see page 195).

Opponent: You can’t move through a square occupied by an opponent unless the opponent is helpless. You can move through a square occupied by a helpless opponent without penalty. Some creatures, particularly very large ones, may present an obstacle even when helpless. In such cases, each square you move through counts as 2 squares.

Ending Your Movement: You can’t end your movement in the same square as another creature unless it is helpless.

Overrun: During your movement, you can attempt to move through a square occupied by an opponent (see page 201).

Tumbling: A trained character can attempt to use Acrobatics to move through a square occupied by an opponent (see the Acrobatics skill).

Very Small Creature: A Fine, Diminutive, or Tiny creature can move into or through an occupied square. The creature provokes attacks of opportunity when doing so.

Square Occupied by Creature Three Sizes Larger or Smaller: Any creature can move through a square occupied by a creature three size categories larger than itself.

Barring an ability that grants a specific exception, a small Dire Rat could only share a square with a foe that is Tiny or smaller (the rat could pass through a Huge or bigger foe's space, but still can't end its movement there).

Now, if you are somehow in the same square as your foe, you can not grant or receive a flanking bonus from the default flanking rules:

Flanking wrote:

Source PRPG Core Rulebook pg. 197

When making a melee attack, you get a +2 flanking bonus if your opponent is threatened by another enemy character or creature on its opposite border or opposite corner.

When in doubt about whether two characters flank an opponent in the middle, trace an imaginary line between the two attackers’ centers. If the line passes through opposite borders of the opponent’s space (including corners of those borders), then the opponent is flanked.

Exception: If a flanker takes up more than 1 square, it gets the flanking bonus if any square it occupies counts for flanking.

Only a creature or character that threatens the defender can help an attacker get a flanking bonus.

Creatures with a reach of 0 feet can’t flank an opponent.

Since the rat is inside the foe's space, any line from its center can't possibly pass through opposite borders.

As such, you'd need a specific ability to enter your foe's space and a specific ability to be treated as flanking from that location (as noted in the previous post): See the Swashbuckler Mouser's Underfoot Assault deed for an example of an ability that specifically does both.

[Baba Ganoush]

If the AC is reduced to tiny or smaller it can occupy the same square- (From Big and Little Creatures In Combat under Combat "Creatures that take up less than 1 square of space typically have a natural reach of 0 feet, meaning they can’t reach into adjacent squares. They must enter an opponent’s square to attack in melee.") <- however, it won't contribute to flanking.

If the Dire Rat can obtain the swarming trait it could take Underfoot (BotB version)

Not RAW but many tables allow a creature to occupy a square with a prone creature (in addition to helpless creatures). If that is the case it could possibly attempt to tumble through the enemy's square, stop in the enemy's square & then fall prone in the square. Or it can start out prone and use Full Round Action to tumble 5' (at -5) "through" the enemy's square (& with only 5' movement) end there - or crawl into the square while prone (provoking AOO).

Would the Dire Rat flying or levitating in the 5' cube ABOVE the enemy give the desired effect?