I think that the huge majority of Americans would think that
Mattie was basically a female name. I know I was very surprised to find out that the recently famous young poet
Mattie Stepanek was a boy instead of a girl.
Carson is well-known as a female name in the South and to literature professors because of the author
Carson McCullers. I would definitely assume that Codi and
Jordi, in those spellings, would be girls.
Shea has been regularly used for both genders in the USA for years. I also would not assume that Josia and
Josiah were pronounced the same way.