I had always been taught that the souls under the altar, chapter 6, were from the entire church age, but that the blood washed saints in chapter 7 were only from the last
3½ years before Jesus returns, i.e. the great tribulation. It says that they came out of great tribulation, which I was taught that this means "the great tribulation", not "much tribulation" like in:
Acts 14:22 Confirming the souls of the disciples, and exhorting them to continue in the faith, and that we must through much tribulation enter into the kingdom of God.
  or "tribulation" like in:
Revelation 1:9 I John, who also am your brother, and companion in tribulation, and in the kingdom and patience of Jesus Christ, was in the isle that is called Patmos, for the word of God, and for the testimony of Jesus Christ. 
    That these cannot be inclusive of church age saints, but the souls under the altar are.