First and foremost, the Bengals will more than likely sign him for a short term deal that is incentive laden, very similar to the Terrell Owens deal in 2010. Harrison has always played with a chip on his shoulder ever since being cut by the Ravens in 2003. Lots of people, myself included, thought the Steelers would find a way to keep Harrison on their squad so he could retire as a Steeler, however, we know how this business works and it is just that, a business.
As far as mixing Harrison into the scheme of things, I’ll let Mike Zimmer worry about that. Zimmer has done an excellent job molding the Bengals into a top 10 defense. Consider this for a second; It is late in the 4th quarter, and the opposing offense is facing a third and long. Zimmer decides to rush 5 and drop 6 in coverage. Think about the possibilities. The front four rushing the passer could be Johnson on the end, Atkins in the middle, move Dunlap one spot in to now rush as a defensive tackle, and Harrison on the edge. That is an extremely intimidating pass rush to think about if you are Joe Flacco or maybe even big Ben.
Harrison has gained criticism as of late for not being the best in coverage, and playing out of position in the 4-3 will not help. I am certain he will only be used for 20-25 snaps per game and I am sure he will do just fine in what limited pass coverage he is required to do on the field. Again, I leave this one to Zimmer, and the coaching staff.