Who CAN Congress Subpoena?

Last night on Fox News' On the Record with Greta Van Susteren, Attorney George Felos - the man who has passionately represented Michael Schiavo (a man who, while still married to Terri, had children by ANOTHER WOMAN!) in his attempt to have his wife die a slow and painful death based on HEARSAY while claiming this an issue of "Terri's rights" - complained about the idea of Congress being able to subpoena ordinary Americans. So, what precisely makes someone an ordinary American by George Felos' definition? Does it depend on their income? Does it depend on their profession? Does it depend on their health? Does it depend on their age? Does it depend on their class? Does it depend on who they know? What, precisely? Congress has subpoened baseball players (in fact, they wasted time talking to baseall players when they should have been dealing with Terri Schiavo's case - the steroid issue, while important, is NOWHERE near as immediate). Congress has subpoened corporate executives. Congress has the right and SHOULD EXERCISE AND RETAIN THE RIGHT, AS CIRCUMSTANCES REQUIRE, to subpoena ANYONE. If Congress is prevented from being allowed to subpoena "ordinary Americans," ANYONE can claim to be "ordinary" and thus not have to appear for a Congressional summons. At that point, Congress will no longer be allowed to subpoena anyone. This is stripping away a power from Congress that they must retain. And if local judges, such as Greer, have the power to nullify a Congressional subpoena, as has been demonstrated by Greer (the man who made a ruling equal to a harsh death sentence to Terri Schiavo after she committed NO crime), then again, the Congressional subpoena is devalued. I urge Congress to retain their power and exercise it in this case. It is really quite sad that a local judge can defy the United States Congress.