Well both the Centauri and the Narn (at one point) were engineered by the Vorlons to produce telepaths.
But left to their own devices, different species with the same potential could still develop differently. Evolution, whether shaped by natural selection or by technology and culture, is still affected by circumstances, so there's no reason to be surprised that different species developing independently would end up developing in different directions. A "godlike superbeing" adaptation isn't necessarily "more advanced" than a sophisticated "mere mortal" technical civilization; it's just a different way of adapting, with the advancements more internalized within the body and brain than externalized with technology. So there's no reason to assume that Narn and Centauri were in any way "arrested" in their development just because they chose a different path.
Plus the Ralgan that looked like Franklin specifically said they were evolving, not "we were engineering ourselves." So it's not conclusive either way.
Strictly speaking, evolution just means change and adaptation in response to situational needs. There can be evolution driven by random mutation and natural selection, but there can also be evolution driven by genetic engineering and social selection. Some people or cultures might not be willing to define artificially engineered biological change as evolution, but others, particularly those that actually practice it, might choose to define it that way.
I think there's an intentional ambiguity as to exactly how a species can make the leap to the "First One" level. It's intrinsically beyond what we think we know about evolution. I think it's even stated in the Psi Corps books that while science can verify the existence of telepathy, what happens to the brains while they're being scanned, the gene that marks psi potential, the actual mechanics of how and why it works totally resists analysis.
Not that it's magic by any means, but like the hyper-evolving phenomenon it's so far beyond what a "younger" race can understand it may as well be.
Oh, the way psi powers were depicted in B5
was pure magic, no matter what the claims in-universe. Psi powers in general are a fantasy idea grafted into SF, all too often used as an excuse for bypassing the laws of physics -- if something impossible happens, you just say "Oh, it's psionics" as if the fact that there's a Greek word for it somehow counts as a scientific explanation.
For that matter, B5's approach to biology and medicine in general was pure fantasy. All that nonsense about "life energy" transfer healing people or extending or shortening their lifespans, as if longevity were a gas tank that we burned through at a steady rate. I always hated that. B5 made a good effort at plausibility in certain aspects of physics, but when it came to medicine and biology it was some of the most ludicrous fantasy I've ever seen.