Its not "that" much of a stretch, honestly. Tybalt, the prince of cats, the deft bladesman, is a fairly logical choice to become a ninja. While we can giggle about him looking similar to Sasuke from Naruto, he's much more... sensible than, say... turning Juliet into girl, hiding underground as a boy, defending the city as a caped crusader.
Tybalt, beyond the spoilers of what actually happens to him, is a pretty good translation.
He's a badass with a blade = Check
He really, really hates Montagues = Check
He's protective of Juliet = Check
So, basically, what I'm trying to say is that I am more than okay with Tybalt being a ninja.