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If the AC worked with 14" Flex, then 14" round metal will have less restriction because it is smooth inside. So you are probably correct with the size. If you want to be sure that the critters stay away from the insulation, you can get sections of 20" metal duct to put around the insulation and seal it with tape. If you insulate any metal duct with bubble wrap or fiberglass duct wrap, then you will always have that same concern. The best critter repellant is to get something they don't like the smell of and place it around the duct work. Moth balls are offensive to some critters, find out what type critter and what smell they don't like, then put that stuff near your stuff that you don't want the critters near.

Just don't tell EllieMay that you don't like Critters

As far as cutting the 5 Ft pieces of pipe to a custom size, you will be better off with offset snips because your hands stay above the work. With the standard snips your hand is between the upper side of the sheetmetal cut to the left and the lower side of the sheetmetal cut to the right. right between the two razor sharps edges of the metal. and when you cut a 14" round duct, that is over 43" of cutting that edge. A person can end up with lots stitches if thy are not careful. Have band-aids ready. If you need to use the flat cut end to go into a 90° elbow you might need a crimping tool also.


Also, to reduce noise, the air flow should go from the flat edge opening to the crimp end. Try not to make a pipe or a fitting with a crimp end on both ends of the same piece.

Here is a set of both right and left snips from HD. You may find that only right snips will not get you everywhere you need to cut. https://www.homedepot.com/p/Milwaukee-Left-and-Right-Offset-Aviation-Snips-2-Pack-48-22-4523/302716388