Asking potential tenants questions about their credit report

Are there any questions you cannot ask a potential tenant about information based on their credit report?

If there are 2 potential tenants and you run reports on them, can you ask potential tenant A something about potential tenant B's credit report?

If you run a credit report, but later decide not to rent to the potential tenants based on something not on their report (although you did run one) do you still have to send them a letter?