Touring Plans recommend Fast passes for different attractions. Is that based on the waiting in my plan or the attraction? Will there always be suggestions for fast passes? Will this list change if I get other fast passes?
I am pretty sure it just recommends for the rides that have the longest wait on your plan. On the TP website, at the section for each park there's a menu item on the sidebar called "Fastpass", if I remember right, and it gives more top-level advice for which attractions are the best use of FP if you're looking for more of an overview.
It will always suggests FP's if you have the settings set that way, as long as you tell it you are eligible to book 3 FP's or whatever it will give you suggestions.
If you put in one that you've booked and then re-optimize, it may update the recommendations based on the new TP it generates.
Here is right from touring plans and what they look at.
I know that it is based on information that they have gathered and what is most requested and the waits. I would follow what they suggest, it is very good information and will be a great help for you.