When are Keeper actions available?
This is the answer from Loney Labs (Alison Looney)


  We would rule that you can use the power of a Keeper as soon as you
  play it.  There's no way it would depend upon keeper or hand limits,
  or play rules.  Your turn is not over until you have taken all
  optional extra actions, including those based on special Keeper powers
  and extra actions on Rules.  So, as soon as you play that Keeper (or
  that Rule) you can do what it says you can do, because your turn is
  not over until you have taken (or decided not to take) that optional
  extra action.
  
  It may happen that a Keeper power has no effect at a given time. 
  Example: in Star Fluxx, you could steal a crew member with the Captain
  as soon as you play the Captain - but if no other players have crew
  members on the table to steal, then that's not going to be an option. 
  Another example: In Zombie Fluxx you could kill a Zombie with your
  Shotgun as soon as you play the Shotgun, but if there are no Zombies
  to kill, obviously nothing's going to happen.