I think it should be remembered that Kosh admitted he was wrong and was willing to sacrifice himself to correct that error. Not something I can ever see Ulkesh doing.
And yes, Kosh and Ulkesh were contemporaries, though Ulkesh was
the more military of the two, very isolationist, while Kosh was the
curious one, interested in the younger races, and more willing to
extend himself (with sometimes unfortunate results). Kosh always
worried what Ulkesh would do without his moderating presence...and
ultimately had to be the one to take him down to allow the younger
races to step forward.
I think what some people miss in "Into The Fire" and quite understandably by the way it was presented is that Loiren let the whole fleet saw the Vorlons and Shadows "private" discussion with Sheridan and Delenn. Any doubts about their moral authority disappeared when the whole fleet crowed around Whitestar 2, willingly sacrificing themselves. If they'd have had the budget for it, it might have helped to have been able to see the other races on their ships...or not, as it would have split the focus of the argument.