I really liked the appearance of the Thallonians in The First Virtue
; it added some depth to see a different writer handling them. Wasn't Picard sparring with a Thallonian for control of some sector, as I recall?
I used to run a PBEM RPG where one of my players was a Thallonian, and I typed up a set of notes based on all the Thallonian novels up to that point (this was 2002 or so). I should see if I still have those somewhere.