I think such a tree would depend in large part on what approach one wants to take. Do you want to learn probability to get a formal foundation of probabilistic reasoning? As far as I know, no other rationality skill is required to do this, but a good grasp of mathematics is. On the other hand, very few of the posts in the main sequences (http://wiki.lesswrong.com/wiki/Sequences#Major_Sequences) require probability theory to understand. So, in a sense, there is very little cross-dependency between mathematical understanding of probability and the rationality taught here. On the other hand, so many of the ideas are founded on probability theory, that it seems odd that they wouldn't be required. Thoughts?