The squeeler Bush Hog rotary cutter is a little expensive, but an excellent choice. Unless you have over 40 PTO HP, I wouldn't be concerned about a slip clutch. The "shear pin" is normally a 1/2 in bolt that bolts the PTO drive shaft to the mower gearbox. It is not likely that you will shear one in a year of mowing unless you hit one of those immovable objects. An old tire seems to be a prescription for disaster. I have hit pieces of steel pipe and the cutter will actually cut through the 1/4 in walls. Actually, if a blade comes loose it can go through the side of the mower. Be sure to get the mower with at least the rubberized fabric guards on the front AND rear of the mower.
Yes, you can normally change to the slip clutch, but the expense is not normally worth it unless you start out with it. It is normally only standard on gearboxes greater than 60 HP for protection of the drive line. Unfortunately, if not kept properly maintanined, it provides far less protection than the shear bolt.
Read the instructions carefully when you get the mower. Some installations require you to shorten the drive shaft. Not likely, but it is all covered in the manual.
Each piece of equipment normally comes with it's own PTO drive shaft and they are not normally interchangable since the length is usually different. They are kinda expensive, so taking good care of them saves a lot of grief and money.
The PTO is inherently very dangerous as a shoelace caught in a revolving shaft can break an ankle or worse in less time than you can react. The shields are all there for a purpose and should NEVER be defeated. Turning the PTO OFF before getting off the tractor is the only safe thing to do.
A rotary mower can hurl objects at high speeds and break windows or hurt people. Keep people far away and put cars in garage when mowing. The guards are a good start to minimizing this hazzard.
Probably the Rotary Cutter like you are getting and the box blade are the two most common implements used on tractors.
