Without LE experience he will have a hard time recognizing it. It’s not always obvious and “questionable practices” can easily be rationalized away to the neophyte.
Grennell needs to to be DIR\CIA. That’s more his world and where he can have the most positive impact.
But if Trump does want to go down the path of bringing in someone with a career law enforcement background I certainly think someone who is (1) not a federal prosecutor (2) is a real cop and (3) is not from the FedGov is the way to go. So a guy like David Clarke would be good, if he'd do it. My concern is that such a person would have too much affinity and respect for the agents and the institution and would be reluctant to take a wrecking ball approach. That's what's needed.