{According to my interpretation of the rules, a Tiny creature within the space of a Medium creature is considered "adjacent" to that creature, and since 4 Tiny creatures fit in a single, Medium square, it's relative; ergo, Wingtip would be flanking with any other PC adjacent to the creature whose space he would be sharing, but the other PC would not be considered flanking.
Does this sound right?}
Yes I am Adjacent to any creature I share a square with as part of the rules more specifically it's under the pixie's description.
This is how Flanking reads in the Compendium. Based on this since I'm not on an opposite side of the creature I share a square with I would say no Flanking bonus. It's a cool idea and all but it would feel cheep for me to use it even if I could as part of the rules.
Flanking
One of the simplest ways for two allied creatures to gain combat advantage is for them to take up flanking positions adjacent to an enemy.
Combat Advantage: A creature has combat advantage against any enemy it flanks.
Opposite Sides: To flank an enemy, a creature and at least one of its allies must be adjacent to the enemy and on opposite sides or corners of the enemy’s space. When in doubt about whether two creatures flank an enemy, trace an imaginary line between the centers of the creatures’ spaces. If the line passes through opposite sides or corners of the enemy’s space, the enemy is flanked.
Large, Huge, and Gargantuan Creatures: A Large or larger creature is flanking as long as at least one square of its space qualifies for flanking.
Restrictions: A creature cannot flank an enemy that it can’t see. A creature also cannot flank while it is subject to any effect that prevents it from taking actions. If no line of effect exists between a creature and its enemy, the creature cannot flank the enemy.