LionFish says.......
Nothing! As of this point why do you need to get any fish? If you are upgrading in a month then just wait until it cycles to get a new fish. It makes no sense to crowd a 45 gallon for a month when you could just be patient and wait till ya get the other tank up. Just my opinion. But if you want a fish for your 75 gallon you could get a snowflake eel or Dwarf Lion.