With that example of the KNF, and this is just an example and it doesn't really say were planning on asking the Bretonians for contracts, what we hope to do is set a contract for several months at a time. Nearing the end of time, the people who we work for can choose to either renew the contract or just let it die. By the time it would end Kusari would hate us, therefore we would most likely move elsewhere, putting more ships into other contracts.
After a few months we would move back to Bretonia and offer them another contract.
That Destroyer will not be requested for a long long time, and I hope it is thought of as it's own ship instead of a Zoner Destroyer. Although I will request the blueprints for the Destroyer and role-play our engineers altering it for our own needs.
I don't think the Bretonians would be too happy (IF that scenario happened...), especially considering you were with the Kusari before, killing Bretonian pilots. If you make a contract, I think you should make it permanent.