Actually, Congress is acting in an investigatory role here, so the Federal Rules of Evidence don't really apply, just as they wouldn't apply in a grand jury proceeding. Congress isn't putting Clemens on trial, it is investigating MLB and the veracity of the claims in the Mitchell Report. What would the result be? If Congress doesn't think MLB is doing enough to combat PEDs, they may revoke MLB's anti-trust exemption or they may enact legislation requiring random drug testing in professional sports. The only thing they can charge Clemens with is contempt, if he lies to them.
So while this evidence might not make it in if this were a civil or criminal trial, it can be considered by Congress and could also be considered by a grand jury in determining whether to indict Clemens of a crime.