My question is two fold, one what do you deem to be a special circumstance? And two what do the psychic discipline talents do once you have 3?

Using the core rulebook it's impossible to get more than three psychic disciplines, with the addition of Into the Storm and Transubstantial Initiate you can start with an extra discipline, does this mean the one at rank 6 is worthless?

With the addition of Navis Primer and Colony Choir you can start with an extra one too, bringing your potential total up to five.

If you are a Transub from a Colony Choir what happens when you buy the rank 3 and rank 6 psychic disciplines? Do you get the Talented (Any) talent as per the rule in the core rulebook for duplicate talents or is it a waste of XP or does it let you switch your discipline?

Edit: Note also that well known homebrew Legends of the Expanse gives Astropaths another Psychic Discipline at rank 10, with no explanation which makes me think that I'm missing a rule here regarding it.

It may be worthless, yes. The ruling is to prevent Astropaths from having some of everything. The "extraordinary circumstances" should be more accurately called "The GM let me do it". Alternate Ranks in games have never been balanced, so yes you can buy your disciplines early and it results in dead time later.

Most people get epic powers in Legends of the Expanse. By that point another Discipline isn't that special compared to some of the rest of what will be going on.

Ah, fair enough then thanks for the information. A line or two on the Astropath alternate ranks would have been nice to clarify.

My GM for the Astropath game ruled that it becomes a 500 technique instead which is very nice.

It would be good if Legends of the Expanse did comment that this expands the normal rule to rule.