I know there are details regarding the Tyrantine Cabal in the Core Book and some of the Haarlock Trilogy books (Spectre Cell 17 and Inquisitor Herrod) but are there any other sources of info regarding it and it's agents?

The first ten pages or so in "Disciples of the Dark Gods" are about the Tyrant Star: they aren't written as the usual description texts but as fragments and extracts of "official" documents and testimonies; perhaps there's something you can find helpful.

The IH features a careeor option called "Tyrantine Shadow Agent". I do not know it by heart, but it might include something of value to you.