Oh there's no question Londo and G'Kar were the stars of the show, almost a comedy act, and as you say one was never sure who was the straight man and who the comic. The scene at the lift in "Signs and portents", a very heavy episode with just the one bit of comic relief, is incredibly well-written, especially when you know what happens later. And yes it's hard to decide who is in the right and who is in the wrong; the balance shifts all the time, just like the various races are shown alternately as good and then bad, if such simple concepts can be applied to a show as complex as Babylon 5.
Jerry Doyle will surprise you as season three and four get going, believe me. He may seem a little stilted but he really comes into his own. Talia I never liked: she always sounded like she was going to burst into tears, and paradoxically looked at everyone with rather an icy stare, as if they were insects. Bleh. You DO know what happens later, yes? If not I won't spoil it for you...
And yes, once you see the role he was required to play, and compare it to the more action-hero role of Bruce Boxleitner in later seasons, you can see that it's all a question of style. I've often likened them to an old grizzled veteran and his hothead son, and the roles each play are perfect for the men they are.
Thanks for the comments: how far are you into the season now?