Yep, basically when Rip was shut down in his proposal that he be allowed to stop Vandal Savage in the past before his world conquest, he opted to hijack a ship and go rogue, but he still is operating under the principles of minimal interference with the time line... thus he cant grab headliners like the Green Arrow, The Flash, or even Superman, Batman, or Wonder Woman, who all have serious impacts to major world events. The characters taken typically have minor roles in DC events, save for Hawkgirl and Hawkman, who are integral to stopping Savage, and who reincarnate anyhow. Death in time for any one of them would not affect things drastically
One of the discussed principles of time travel here is that Time wants to happen and will correct minor changes... the anti butterfly effect, if you will. The concept is that, while time can be changed through great effort. most small things will be compensated for. When Rip looked through the annuls of history, there was nothing the Atom or Firestorm did that cant be compensated for with another hero. The Thieves are just that... they don't have a major impact if they are lost... The Canary was dead already, replaced by her sister, , and retired from everything to live in seclusion...