So, for people who are confused about the situation, Nobimaru is the most capable shapeshifter among the foxes. Most foxes can only manage one (human) form and are specialized in other fox magic. This is most obvious with Kaede, who isn't even able to maintain a human form anymore. Changing his (human) gender is just part of Nobimaru's repertoire. The other foxes can't do it.
What Nobimaru is probably thinking of doing is try to test the barrier with actual females, his gender-changed self, and a crossdressed male, to see which would be allowed to pass by the barrier. Given the sometimes whimsical and often just plain dumb way kemono magic works, he might have a point here.
The way hilarity usually ensues in this manga is to have the main team also crossdress their pretty boys and try the same trick. My guess is that both sides will manage to pass three members each. The foxes will have the two girls and Nobi pass, the heroes will have Kon, Mayo, and Akira pass.