Book identification - Interstellar humanity enslaved by lizard like aliens

What is the name of the book or author in which:

The action for this book occurs on a human colony world, not Earth itself.

Interstellar human civilization has been enslaved by an alien race. The alien race is reptilian (although superficially very similar in description, these are not the Aann of Alan Dean Foster's Universe). The reptilian race is faster & stronger than humans. I kinda think their race name begins with an 'R' but think I might be confusing it with the Ryall of Antares Dawn series.

Some human soldiers were genetically, chemically, cybernetically, and trained to be super soldiers to take down the reptilians in one-on-one combat. Humans lost the war long enough ago that the remaining super soldiers are old men now. They have been training other humans to fight the lizard aliens in secret - but without the advanced technology, these trainees can only hope to lose more slowly to the alien soldiers.

Also before its fall, humans cached 9 battlecruisers in a last ditch tactic to surprise and slow down the alien advance by reactivating them behind the reptilian lines. Unfortunately for humans, the aliens advanced far too fast to use this stratagem.

During the course of the book, humans on the colony world (on which the action takes place) discover that the reptilians are also in a war with another race (I think they were butterfly shaped but could be wrong). Humanity decides to activate the battlecruisers and get them to this alien race (in an "enemy of my enemy" gambit). Much of the book is about trying to get those 9 cached battlecruisers to this third (butterfly? shaped) race.