Only the bonus disciplines from your order need to be from your order's disciplines
In other words, you are free to choose any psionic discipline from any order, except for the two bonus disciplines you get at level 1 from your order.
From Psionic Disciplines, pg. 3:
At first level, you know one psionic discipline of your choice. The Disciplines Known column of the Mystic table shows the total number of disciplines you know at each level; when that number goes up for you, choose a new discipline.
There is no mention of restrictions, so there aren't any. Compare that to, say, the Bonus Disciplines feature from Order of the Nomad (they all have similar wording, I'm just picking this one as it's in your question), pg. 7:
At 1st level, you learn two additional psionic disciplines of your choice. They must be chosen from among the Nomad disciplines.
Yes, it does also say "of your choice", but it specifically calls out the restriction of being from your order.
Also consider the Order of the Soul Knife. It doesn't have any psionic disciplines associated with it, nor does it get these two bonus disciplines (which every other order does) at 1st level. However, an Order of the Soul Knife Mystic would still get to learn disciplines as per the number of known disciplines listed in the Mystic table; this is because Mystics can choose anything when the level up, the only exception is their order's bonus disciplines (if they're anything other than an Order of the Soul Knife Mystic).