Can you store a Bag of Holding or a Handy Haversack in a Pathfinder Pouch? My idea is, that a PC would like to travel light but still have access to their gear.
A number of spells and magic items utilize extradimensional spaces, such as rope trick, a bag of holding, a handy haversack, and a portable hole. These spells and magic items create a tiny pocket space that does not exist in any dimension. Such items do not function, however, inside another extradimensional space. If placed inside such a space, they cease to function until removed from the extradimensional space. For example, if a bag of holding is brought into a rope trick, the contents of the bag of holding become inaccessible until the bag of holding is taken outside the rope trick. The only exception to this is when a bag of holding and a portable hole interact, forming a rift to the Astral Plane, as noted in their descriptions.
So you can nest bags of holding (and variants of it) without issue, and since the pouch functions identically to a bag of holding (as per its description) you can nest with that as well. You can't remove items from a bag while it's in another bag (just like with rope trick) but you can just access the bag by removing it from the bag it's nested in.
Note, however, that the Pathfinder Pouch's weight limit still applies, so some of the larger bags will be unavailable unless their weight is somehow reduced (e.g. via the spell lighten object or being made of the special material Darkleaf Cloth or both).