Looking at probability travel in the Complete Psionics Handbook, it is almost exactly like a combination of astral caravan/astral traveler. So simply adapting those two should work.
I was looking it over also and it spawned a few thoughts. First, probability travel & astral caravan take an awful long time to navigate the astral plane to get to a different location on the same prime material, it would be faster for them to swim and with swimming or navigating the astral they would run into the same issues managing their goods.
Second, the teleport spells are faster and more efficient and considering that they could be transporting 7-12 slaves in a shot, a greater aboleth would not have the manifester level to transport them in one shot, thus the need for the noble aboleth.
Third, the greater aboleth, if given teleport would have a range of 100 miles per manifester level plus 1 medium creature + 1/ 3 manifester levels. Even if the noble only had teleport, it would have sufficient range and capacity to teleport the slaves and the greater aboleth. Granting the noble greater teleport eliminates the range limit and chance of showing up off target.
Finally, probability travel/astral caravan gives the added ability to go to different planes and prime materials (I believe within the same crystal sphere, but I am not sure). This seems to be more of a function needed for a ruler or grand aboleth looking for a new world to rule.
The nice aspect of astral passenger and astral caravan, is that as psi-like abilities, they stack nicely and you do not have to remove PLAs from more powerful aboleths. If we go this way, I would be thinking along the line of:
Greater - astral passenger (at-will)
Noble - astral passenger (at-will), astral caravan (1/day since it takes approx. two weeks to make it to destination)
If we were to go the teleport route, I would say:
Greater - teleport (3/day)
Noble - greater teleport (3/day), astral passenger (at-will)
Ruler - greater teleport (3/day), astral passenger (at-will), astral caravan (1/day)
You could also just leave the greater alone, as the noble has the mojo to do it all. Thus it could likely be a service that the greater would owe something to the noble for service rendered.
Of course, for something that lives thousands of years, what are a few weeks?
Bykov.