Do you recommend a specific whip to use when cleaning duct board material duct vs. Metal duct?

Answer: All air whip systems recommend 12+ cfm and usually up to 175 psi.
The cfm requirement is the same for cleaning softer walls BUT by lowering the psi on the air compressor combined with adding more tentacle legs lower the impact of each tentacle / leg on impact with the wall thereby causing less damage.
8 legs divides the impact by the cfm and the low pressure designed tubing is softer so still whips around in the lower pressure use.
You will have to experiment with the pressure setting to find out what cleans the best for your job wall softness.