I think John the baptist being a martyr though is sketchy, first off he wasn't a Christian, so unless we count all of the people in the O.T who died for their faith than no dice. And, he was not killed specifically killed because of his faith.
Correct. He was imprisoned because of his preaching against Herod that Herod found offensive, but was killed because of an oath Herod swore in haste. He didn't actually "follow" Jesus but announced his arrival. John had his own disciples too. They followed John while John did his mandated call but none of them (including John) actually "followed" Jesus.
If we look at Stephen, however, he truly followed Jesus and preached the gospel and was killed specifically because of his allegiance to Jesus and His teachings. The scripture says that when Stephen saw Jesus he saw Him
standing at the right hand of the Father, which some have indicated that Jesus, though we know He went to Heaven to
sit at the right hand of the Father, intentionally stood because of the fact that Stephen was the first follower of His that was killed for his faith in Him.
So I believe it was correctly determined by the elders to have the martyrs begin with Stephen.