So as well as Augie and
Gus, you could use
Austin as a nn for
Augustine.
Fritz is a great nn for
Fredrick, but I would not use it as formal name, too short for my taste. Besides, I loved a children's book about cats when I was a child. One of the cats was named
Fritz, so I also think of a cat when I see the name.
Fritz Crispin does not flow well, but if his formal name was
Fredrick it would not matter. I don't think you use a nn with a mn.
"You sought a flower and found a fruit. You sought a spring and found a sea. You sought a woman and found a soul. You are disappointed."
"It does not become me to make myself smaller than I am." (
Edith Södergran 1891-1923)