@FredClinton Thanks for reaching out again!
A skeg is typically installed by running a line under the boat and connecting it to the skeg, which, in the case of the Delta Kayaks, is spring loaded. They also state that their skegs offer 'kink free' operation. REI does not sell the skeg, you would have to work with Delta Kayaks to order that item.
A rudder and a skeg are both designed to drop down and help you track in a straighter line when paddling your kayak in the wind. A skeg helps you track straighter by creating resistance in the stern of the boat equal to the resistance provided by the bow wave at the front of the boat. You can adjust the amount of the skeg you are using depending on how much the wind is affecting your ability to track in a straight line (known as wind cocking).
The rudder, however, moves side to side and compensates for the wind by turning the boat into the wind and creating a straight tracking line. A rudder is a much more intuitive way of tracking straight in open water and can also help you negotiate turns, particularly if you are new to open water paddling.
All of that is to say, if you have a rudder, you do not need a skeg. A couple of advantages to a skeg are that there are fewer wires to contend with (thus there is less likelihood that something can go wrong with the mechanism) and they do not create additional drag from the wind when not deployed (the rudder sits like a small sail on the stern of the boat).
If you have a lot of experience paddling in the ocean and the skills needed to maintain a straight line with a skeg it can be a great feature to have on your kayak. However, a rudder will give you a more intuitive function for tracking straight in the wind as well as providing assistance in turning if needed.
Hopefully this helps!