[lolc]

In the end your metabolism will have its own way of digesting which may or may not be close to the very general glycemic load. Which is compounded by your metabolism reacting differently based on time of day, what you ate before, what after, whether you're sick or just in a different state.

With that out of the way, of course it depends which things you eat together. Potatoes are high on the index but if you combine them with cheese (mmmm cheese) it will slow down digestion and thus lower the glycemic load. Similarly, bread has a high load. But add peanut butter and you slow it down.

As a general rule: Yes, add fat to slow digestion. There may be methods to calculate the load of a compund meal but don't count too much on them. The glycemic index is an average of how people react to foods. It's already impractical to study all the meals in that fashion and still you wouldn't know how you individually reacted to them.

Edit: If you want to watch how your body reacts, you can get a set of Freestyle Libre sensors from Abott which allows you to monitor your blood glucose levels. They're not super accurate, but they easily allow you to track up/down movements which is enough to see how long it takes you to digest a meal.