Here a short tutorial to create a ghost, that can use the nuke ability instantly. First off start removing the requirements of the nuke as described above. Then go to the "Ghost - Nuclear Strike" abiltiy. There you have to change the fields "Effect - Calldown Effect" to (None), "Effect - Effect" to "Ghost - Nuke (Persistent)". Now go to Effects and search for "Ghost - Nuke (Persistent)" and change "Effect - Effect - Final" to (None). If you dont do this your Ghost will commit suicide right after the nuke detonates. At last delete the "Effect.nukeP.Start" in the "Ghost Nuke Launch" actor, to avoid launching the nuke at the position of the ghost. I think the problem is the Ghost - Nuke (Calldown) effect, which requires a nuke to be built and is therefore always linked to the "Ghost Academy - Arm Magazine (Nuke)".
@AimSC: Go I found another thread via the handy dandy search box.
You can find the original article here: http://forums.sc2mapster.com/development/map-development/1332-removing-nuke-requirements/
@AimSC: Go
Why don't you leave the call-down, set the cast to something short like 0.5 seconds, and from there, just add a CD to the ability.