Your details remind me of the Clutch Turtles from the Sharon Lee & Steve Miller Liaden Universe series. These details are from the Wikipedia entry:
These non-humans are even larger than Yxtrang and very long-lived; they appear much like turtles walking upright, hence the name. The length of their names are directly proportional to how old they are and their accomplishments: Edger's full name apparently takes some hours to recite. They are usually slow to act but are very dangerous when angered.
Clutch turtles travel in starships made from hollowed-out asteroids using an electron substitution drive that can have hallucinogenic effects on the human nervous system. They are able to command forces of great destructive or healing potential by singing. Clutch turtles are greatly feared and avoided by the Yxtrang as the result of a resounding defeat in battle many years prior to the timeframe of the story.
The Clutch turtles encountered in the "Agent of Change" sequence make up a "market research" team on behalf of their clan, who are known for manufacturing crystalline blades of extreme sharpness and durability by growing them in caves over a timespan of decades.
This clan of Clutch turtles manufactures crystal blades of exceeding sharpness and durability; these particular Turtles are named for the function they perform in the manufacture of their crystal blades. Much more information can be found at https://en.wikipedia.org/wiki/Liaden_universe and at the Authors' own website, http://korval.com/ as well as the website where they post additional information and stories, http://www.SplinterUniverse.com/.
In the first book, Agent of Change, we learn that these specific Clutch Turtles are indebted to Val Con yos'Phelium of Clan Korval and have accepted Val Con as part of their clan. (That story is told in one of the chapbooks the authors have written to provide additional information/stories set in-universe.) They provide him with their spacecraft so he and his companion, Miri Robertson, can escape the people (the Department of the Interior, which is dedicated to taking over Liad) hunting him. We later learn that the race of Clutch Turtles is strong enough to make a galaxy-wide crime cartel, the Juntavas, fear for their existence when they "persuade" the Juntavas boss-of-bosses to issue a galaxy-wide amnesty for Val Con and Miri.
Clutch Turtles appear in several (but not all) books of the series and are mentioned in others; they have a supporting role in the stories.
Even if this isn't the series you are trying to identify, it is an excellent series. I recommend it for your reading enjoyment.