I figured that telepaths had not been allowed, or really wanted, inside the Rangers for a number of reasons.
o Human telepaths were part of the PsiCorps, which causes loyalty issues.
o Centauri telepaths are owned by the Houses and act like slaves, again, loyalty issues.
o The other race telepaths have pretty much the same thing going for them, loyalty and organizational issues.
o Plus, and keep this in mind, the Vorlons were their and they have a lot of influence on telepaths.
Out of all the races only Minbari telepaths do not have the same problems as the other races do. Minbari telepaths are always Religious Caste. Religious Caste can be Rangers without problem, plus its very rare for a Minbari to turn traitor - though it does happen. Still, there are not that many Minbari telepaths and they have a lot of functions they preform for the good of the state.
Once the ISA exists for a long time and people have loyalty more to it then their own people, I figure that the recruiting of telepaths can happen without problem.
Also, do remember, that by 1 million years hence all of humanity is telepath. 1 million years is far less than the few billions taken by the Vorlons, this means a lot of changes and stuff happens over brief periods of time.