Kanda is more pragmatic than you take him for! Who he'd let watch his back in a fight is not the same thing as who he'd tolerate as company on a desert island. Kanda will let any Exorcist watch his back, with the current exception of Allen, because the 14th thing is still a question mark for him in Marina.
Outside of the Exorcists, he gives props to other people with marked combat ability: Jun, Kitty, Saber, Faith -- anyone he's really SEEN in a fight. He wouldn't have let Ichigo do it, but only because he knew Ichigo had a psychoath inside him and severe control issues.