I think this topic could debated both ways. As far as if unusual heritage is fitting to grimm, what kind of heritages should be allowed, and if the narrator gets to choose and implement them. The way I see it, the whole thing really depends on how you're running your campaign, and the people playing. I know if my players were to get unusual heritage they'd want me to implement it as I see fit and when I do I'd get the typical "THATS AWESOME!!!!11!!!1" reaction, as opposed to my brothers group which would likely respond, "...thats a bit over the top don't you think? I mean that'd be nice to have known when I was in 3rd f'n grade!".
From another standpoint, would it stand out as being too quirky to your campaign? If your kids (their characters I mean) have always been struggling with the twisted, dream-like setting of Grimm and their ordinary Real World habits and limitations, then maybe it'd be too much for your players to suddenly be "burdened" with this totally badass (<- my opinion) heritage. Discuss with your players how they feel about when they're creating your character.
Personally, I like it because I know my players would like it. The Guardian Angel heritage and the Artificial Human are sick ideas, although I think they'd be a bit much for me, I do love the idea. When my friend was inquiring about Imaginings and I told him, simply: it does badass things, his response was "...how 'bout SHAZAAAAAM!" (image). So I figured maybe when the normal kid enters the Grimm lands he/she slowly starts to develop the powers of their iconic comic book superhero. Yes, yes I know...that's waaaay overpowered for a bunch of fairy tale characters, but I just thought I'd share. Maybe before you reveal to them their heritage you could subtly ask who their favorite comic character is.