No two animals are exactly the same size, and you can’t count on the form you buy to fit any animal perfectly. Modifying the form is part of taxidermy, and one of the great characteristics of the foam manikin. Easy to cut and glue. I would highly recommend buying some videos before attempting more mounts. That will let you get an understanding of the process, and give you an idea of supplies you’ll need. There is no “best” kit, and you really don’t need a kit at all. Buying supplies separate will allow you to get a form that is closer to the animal you have chosen. A kit may have a fox squirrel size body, which obviously won’t work with a grey squirrel.
This is a bad time of year to be obtaining ducks. They are full of pin feathers and just don’t mount up well at all. Try to get your birds late in the fall or winter. WASCO and McKenzie both have good supplies and videos.