Crowder is very capable of defending all but the quickest shooting guards in the NBA. No, it's not ideal, but he moves his feet well, and will have a size advantage most nights. Hayward can then defend the small forward.
The best defensive team the C's will put on the floor, while still using IT at the point, involves Smart at the SG position. And yes at times he will struggle to score, but with Hayward and IT also on the floor, he'll be left to do what he does best, facilitate, agitate, and defend.
Anytime IT is on the floor the opposition has a path to score, you hope he plays well enough on the other end to off set it.