A CDA has nothing to do with iddle thrust but with not leveling off.
Minimum drag approach is a different thing and means minmum flap and gear up for as long as possible. It does amaze me how some people do ask for gear down 12 miles out and at 3 am...
An iddle thrust descent all the way to 1000', although ideal in terms of fuel burn, can easily degenerate into a rushed approach. I think that iddle descent as far as possible but with partial thrust descent segments in the last 20 miles gives a better margin for error. ATC normally scuppers the plan anyway.
In the UK it is described a good practice to do a CDA whenever possible. (Jeppessen country differences)
The 500fpm minimum rate only applies above the Transition Level.