If their testimony is accurate then I see no problems with them absolutely slamming and damning the administration if warranted.
You wouldn't expect them to stay silent and hide the truth if they were under h. clinton, would you?
Criminal behavior, no, I wouldn't. But personal opinions, yes, I do believe they are supposed to keep out of it.
I understand them "speaking out" but from this article it is hard to nail down anything specific beyond a few vague comments out of context. Imagine Gates speaking out about Washington, Patton speaking out about Eisenhours, McClellan speaking out against Lincoln, MacArthur speaking about against Truman...wait a minute, this probably happened. Generals are all politicians. Once they are retired what else do they have to do but try and get elected? This makes them easy targets. Old soldiers at that rank rarely "fade away".