Wet insulation in flex ducts
Ordinarily, flex duct is connected to collars of the same size. Since the collars are made for flex duct, it is an engineered system and all the components fit together easily. As for 6" metal duct, 6" flex duct is the correct size. If you are unable to slip the inner duct onto the metal by rotating the coiled spring hose (inner duct) while slipping it on, you may need to first crimp the metal duct like one end of prefabricated rigid metal duct sections. The home centers sell a tool for making the crimp. Be sure to secure the inner duct with tape.
As for the cut length, all components are typically cut to the same length. When connecting to a metal duct section, it may be advantageous to extend the cut of the outer jacket. Provided that the flex duct is not taut (there should always be a little slack is total length), you can always pull on it thus shrinking the length of the secured inner duct a bit.
Be careful not to puncture either the inner duct or outer jacket. This will result in moisture intrusion and you will be back to square one.