Congress can subpoena whatever the hell they want, They can investigate whatever the hell they want, and the can conduct the investigation however the hell they want.
There are no qualifications set by the executive. That's not the way it works.
Hmm. Ok. Though I believe the supreme Court has ruled that there needs to be a basis...whether legislative or investigatory. Hence my recommendation that they read the unredacted Mueller report to see where they may find one. Or not