As fibres are products of nature, we can’t design them 100% to our liking; of course the larger fibre length can be (mechanically) shortened and insoluble fibers can be (partly) solubilised. This leaves sufficient challenges for any product developer. For example a known fibre, commonly used in many glutenfree products is Psyllium. Psyllium can absorb a massive amount of water (about 20 times its weight in water) and create a gel. This is great if you are looking for certain stability in dough handling and depositing/ shaping. Within this gel matrix you are even able to bind some oil as well. A big downside to this is that the release of water during heating (cooking, baking) is poor; resulting in breads, cookies and cakes that seem to be undercooked and poor taste.
On the other hand soluble fibres with little to no viscosity changes, such as dextrins from corn/ maize and wheat can have an opposite effect: hardening of the structure. Biscuits can become so hard that they feel like a brick and dipping in tea or coffee isn’t an option but obligatory for consumption…. Not very friendly to everyone.
The best way around this is to balance things out (of course…. But how?). While in the past bakers (and many developers) would just mix the too hard dough with the too soft dough and see what comes out. This is of course a proven empirical method of development it also is quite time consuming until you find your ‘holy grail’ if you aren’t discouraged beforehand. A first indication could be given by the supplier, where they might be able to share with you data on water and oil absorption. This is a good starting point, but then it is about the texture: a cake can be softer and have more moisture content, where biscuits should be dry. In a biscuit psyllium would be difficult to use, but the harder biscuit one could compensate with a insoluble fibre that adds a bit more volume to it. Cakes would benefit from the combination of soluble and insoluble fibres for volume and stabilisation and with a bit of psyllium in the matrix it could result in good moisture retention.