What's your opinion about this?

I'm writing a story and it's details are here:

So, I was wondering, should I have some kind of sorting magic beings into "houses", like the talking hat did in Hogwarts? I was thinking about taking the kids to the propthess, and she would take just a drop of their blood and taste it, and by their blood she would know are they elves, witches… And, because my story takes place in another world parallel to ours, it has different imaginary countries and kings, should I have another type of money there, like with the portrait of the king on it?
I've also considered that that world look like it's in medieval times ( because in my opinion newest technology just doesn't fit with mine seeing of magic ), but it's kinda the same situation in Harry Potter:/
And, for the mailing sistem, I was thinking about butterflies that transform into letters when they come to the hand of the one to who it's sent. What do you think about that?

Or, should I stay away from these ideas, because HP has it's hat, golden money and it's kinda medieval-ish? But yet, if they're in another world, people can't get paid by dollars, and I don't want them to live surrounded by lap tops, iPhones, etc.
Any prepositionon is welcome!

I like the idea of the prophetess. Definitely do NOT do the hat thing, houses are okay, but you could call them something else, like how Divergent is factions. The butterfly letter thing is cool.