Hi I'm new here and I was wondering how you would compare Don Vito, Clemenza, and Tessio from the first and second movie?

I'm not sure what you mean.

None of them were in the second movie, except for the flashback scenes.

Do you mean how they compare from their younger days in the flashback scenes to their later days as depicted in the first film?

I think you might mean, how to compare young Vito, Clemenza and Tessio (in GFII) vs. the mature Vito, Clemenza and Tessio (in GF).
Young Clemenza is cocky, brash, loquatious. Until the Fanucci murder, he was the dominant partner, or at least acted like it. Note how he bosses young Vito around, especially in a deleted scene in which he orders Vito to deliver some dresses from their theft, while he dallies with a housewife. But, after Vito kills Fanucci, he's humble, and Vito's in charge. You see this clearly in another deleted scene, after Sr. Roberto exits Vito's office. Clemenza introduces young Hyman Roth to young Vito. Note Clemenza removes his hat before speaking to Vito, and refers to him as "Mr. Corleone."

Young Tessio says practically nothing in GFII. But the revealing scene in GF comes when Vito's retired and Michael's in charge. In the scene where Michael's sitting in the Don's chair and Vito's looking at his tropical fish, Tessio leads the complaints about Barzini's encroachments. Finally Vito says, "Do you trust my judgment?" Clemenza replies, fervently, "Always, Godfather." Tessio simply hisses, "yes-s-s-s." He leaves the room in a huff. A harbinger of disloyalty to come.

Not only that, but did both Marlon Brando and Robert De Niro capture the essence of the Godfather? Was Bruno Kirby's Clemenza what you would have expected a young Clemenze to be like?

Brando and DeNiro fulfilled my expectations based on the novel. But then again, their performances were so iconic that it's impossible for me to think otherwise.
Bruno Kirby's performance was a lot less iconic, and his Italian wasn't very good. But he did provide a certain amount of energy that I inferred from Puzo's description of the young Clemenza.

I agree. DeNiro nailed the younger Vito's mannerism, voice, etc. Kirby wasn't the best physical choice for Clemenza, but he did capture his character. Of course, the novel had Clemenza as "immensely tall" which was ignored in the movie. I always wondered why Puzo described him that way since Italians are not know for their height.

a agree that robert deniro's portrail of a younger vito was as close to perfection in an acting capacity..the guy nails every role he gets..sometimes the choice of film isnt great..but we can forgive him these small lapses in choosing what role he next wants to play..he has played some of the most iconic figures in film obviously young vito; jimmy the gent ;neal mc cauley ; jake la motta..and many more..