I guess I'm not sure what your question is. What kind of further guidance are you looking for? Are you looking for what the prohibited bases are? Are you looking for guidance on what constitutes disparate impact in collections? Are you looking for resources?
You can't discriminate (i.e. "treat customer A different than customer b") based on "race, color, religion, national origin, sex, marital status, or age (provided that the applicant has the capacity to enter into a binding contract); the fact that all or part of the applicant's income derives from any public assistance program; or the fact that the applicant has in good faith exercised any right under the Consumer Credit Protection Act or any state law upon which an exemption has been granted by the Bureau" under Reg B.
I'm not actually sure if FHA applies to collections if it's a mortgage, but I wouldn't treat anyone differently based on Familial Status or Disability either.