Well, by RAW, you could take both the Spider Step and Cloud Step feats without taking the Slow Fall ki power as their prerequisites do not include Slow Fall. But as both feats use half your Slow Fall distance as the maximum, they would not do anything for you until you got it.
As the new Slow Fall ki power does not list a maximum distance, it would probably fall into one of the following possibilities if you have the Slow Fall ki power:
1) Use your Unchained monk level to determine what your slow fall distance would have been before Unchained to determine the maximum distance of Cloud Step / Spider Step.
2) There is no maximum distance of slow fall, so Cloud Step and Spider Step are limited only by how much movement speed you have.
3) As Feather Fall allows you to fall at most 60 feet per round, your slow fall is effectively 60 feet. Cloud Step and Spider Step use half your slow fall, so they can effectively be used for 30 feet.