I like Chuck and I think he'd be good for the Cs - they need some grit and rebounding - but I don't think he's the answer at the 5. They need a real big man, someone who can protect the paint and handle (I use this term loosely) the Howards and Bynums of the league.
Hayes is not the guy to "protect the paint," in the manner that Perkins and Garnett have done for us, but he's capable of keeping guys like Howard and Bynum out of the paint. He has fantastic lower-body strength and works very hard to deny good post position.
We've had surprising defensive success with the KG/Baby pairing in the past, with the primary drawbacks being Baby's terrible rebounding, mediocre defense against quicker bigs and, IMHO, over-aggressive offensive play. Hayes is better in all of those categories.