I am especially looking forward to using a Trygon in this project.
The first thing I think of with a cyborg Trygon are drills instead of claws... disturbing...
Anyway, I thought a bit about it and came up with this, just to see what your ideas were on the evolution of the bio mechanical Tyranids;
The first experiments were fairly basic but proved successful in preserving bio matter, which was the goal of these experiments.
The initial usage of the metals was mostly centered around the skeletons of the smaller creatures, thus saving minerals which could be used for armor and weapons.
The first bio mechanical Tyranids proved to be very effective however and soon more drastic implementations of metals were seen throughout the Hive Fleet; blades were lined with steel, later completely replaced with the strongest of metals, cranial armors were 'forged' (Not sure how you see them shape the metals.) and eventually the Hive Mind started thinking of even more drastic measures.
(This is where they go to the Forge World.)
When the machinery was secured and the Hive Mind had learned of their workings and functions it could reproduce, even improve, basic hydraulic systems, electrical systems and other basics of engineering crucial for the next endeavor.
Within weeks the Hive Fleet produced its first truly bio mechanical warriors, having swapped out several biological joints for mechanical ones and even the weaponry had been improved with mechanical systems.
Each 'generation' the usage of mechanical components became more intricate and made the creatures ever more effective and reduced the bio matter needed for the creation of Tyranids, allowing for an expansion of the fleet.
The new creatures had mechanical eyes, fully mechanical limbs, sometimes hidden beneath flesh, sometimes exposed and the Hive Fleet produced a completely new creature, never seen before:
The Welder, a slightly larger variant of the Ripper, capable of consuming the large amounts of metals which were needed to continue the endeavors of the Hive Mind.
It could use the electricity produced by its four slim limbs to cut through metal, eating the tiny scraps until it could eat no more, after that it would deposit its collection, feeding the Hive Fleet's demand for metals.