Many thought it was madness, foolish even. Others considered it inconsiderate of those lost to the invasions. Yet despite these oppositions, some Rogue Traders have gone about modifying captured tyranid bioforms. Retaining their incredible abilities from their respective Hive Fleets, while some gave them more 'appropriate' forms for various unknown reasons. Whoever started this long chain of heretical experiments, nobody knows. Only that it has gotten so out of hand among parts of the Rogue Traders that the Inquisition gave up trying to track each one down. Merely punishing any dumb enough to reveal any in their acquisition

While many of the traders sought out more 'exotic' types of tyranids for their own companions, you had a different interest. The smaller, more numerous gaunts were far more fascinating, especially with their adaptability for almost any task. It seemed perfect, and the fact few of your fellow Rogue Traders went after them, it was easy to accrue enough for the process. Not to mention that their adaptability seemed to make them more receptive to the alterations you requested.

(Modified heavily after input from a friend on Discord)