20 amp circuits/15 amp fixtures?

If you recall, a few months ago I began a project of re-wiring a dairy barn for conversion into a shop. At that time, I followed all advice given me from this forum. My farm house main panel will be supplying the remote dairy barn with a 100 amp breaker. There will be a 100 amp service disconnect box mounted on the outside of the dairy barn. I was instructed to make sure the jumper (green screw) was disconnected so that the neutral and ground were seperated and not bonded. Since this was not possible with the service disconnect box purchased, I purchased a seperate, insulated grounding terminal from Grainger, which is capable of receiving the #6 ground. This insulated grounding terminal will be mounted seperately, in an insulated fashion, using sheet metal retention screws inside the service disconnect box. At this point, the ground and neutral will have been isolated. From the service disconnect, the 3-#2 THHN/THWN w/ #6 Ground will proceed into the dairy barn to the sub-panel. Currently, there is a 125 amp main breaker in that particular sub-panel box that will be changed out to a 100 amp breaker. The Sub-panel box will have approximately 3 220 circuit breakers and 4 110 circuit breakers. My question is: In the SUB-PANEL box located inside the dairy barn, should not the neutral and ground buss bars be bonded, as in a main panel box inside the farm house? Was it sufficient to seperate them only in the service disconnect box located outside of the dairy barn? I noticed that there was a similar question earlier in the forum, and it appeared similar to my question, except that they did not have the service disconnect box.

how do I install a ground fault plug if there is no ground wire present. Will it ground each plug where a ground fault plug is installed? Is there a book or manual to show how to do this properly.. I can't afford to have a pro do it so need all the help i can get.. THANKS!