I don't really think 'low fantasy' means humans only. I've played some really immersive low-fantasy worlds which still had fantastical species in it. The level of fantasy really depends on how rational the world actually is. If you just went "K I've got a world, let's put magic in it. Oh right, now lets put some elves over here and some dwarves over here", that's high fantasy because there's no real rational thought gone into the world building. If you've come up with logical reasons that these elves exist (perhaps they mutated from humans in an area with a high latent magical field?), and if there's a level of logic to the magic (ie, it's not just "X throws fireball cos fireball"; they have to do something before the fireball will happen), that's low fantasy.