You can't have 2 HOF per decade per the rules. It mentions nothing about where you can't have a team filled with non HOF players or requiring everyone to have 1 of each.
You will draft 2 players from each decade, only one of which can be a HOFer, MVP or Cy Young Winner. So you cannot have more than 12 HOFers, MVP/Cy Young Winners.
My understanding of the underlined is that it's a option of taking 1 HOF per decade not a requirement.