The catalyst playback rate will have no effect on the speed of a video input...

the Catalyst playback rate applies only to movies (ie the speed at which they playback)
So you will see no change in the video input.

The Decklink '1080i 59.94' is 30fps...
59.94 is the frequency of the interlaced fields, which equals 30 frames per second (an interlaced frame is made up of 2 fields)
See: http://en.wikipedia.org/wiki/Frame_rate

Decklink at 59.94 and Catalyst set to 30fps (not 60fps) would be ok...

What exactly are you trying to achieve?

Simon