The Cap who Havok teamed up with in Mutant X wasnt Steven Rogers and that Cap was also a mutant unlike Steve Rogers.
I'm not aware of any substantial prior link between Steve Rogers and Havok, cant see any reason why Alex would be so keen to become an Avenger or any reason why Cap would choose Alex over a dozen other possible candidates.
I think Marvel have alot of work to make this team make sense. Havok and Rogue dont appear to have any vested interest in being Avengers - they dont bring anything to the team it doesnt already have plenty of, neither character is in need of redemption, neither character has expressed any real desire to break from being X-Men. Wanda meanwhile has gone from public enemy number one (it wasnt long ago that the Avengers and Cap were considering doing away with her perminantly) but now she seems to be back on the team no questions asked.
There is going to have to be some explanation to make all of this fit, although with the exception of his stint on the Punisher Rick Remender has done excellent work and I have more faith in him than most writers.
Havok led X-Factor while it was a government team. He's used to being the "public face" of mutants, and understands the politics better than mutants who have never travelled outside of the "X-ghetto".