Your FDCPA claim would be against the collection agency, not the creditor, so at first blush the agency can not bring a counterclaim. Having said that, if the collection agency is smart, it would take the debt by assignment and then counter-claim.
I hope you found this response to be of assistance. This response shall not be considered the rendering of legal advise but instead a general response to a general question. While Avvo is a wonderful resource, nothing can be a substitute for an in-depth consultation with an attorney in the jurisdiction in which the law is to be applied. This response shall not be deemed to create an attorney-client relationship, nor shall it create an obligation on the part of the attorney to respond to further inquiry from the questioner.
Yes. You might be able to sue both the original creditor and the collection agency, if the collection agency is acting as agent. If the collection agency "bought" the account, the original creditor is out of the loop. As to what some unnamed collection agency might do, based upon some unknown facts, I am not a psychic. My crystal ball is broken and out of warranty.
As Counsel Lysell and Lampert point out. The answer is dependent upon whether or not the collection agency is the owner of the claim/account. Without that info. an answer cannot be given.