I like
Frederick and all the nn you mentioned. I think it's a 'charming' name.
Freddy would be really cute on a little boy, but at the same time he could go by his fn or the other nn if he thought
Freddy was a little immature for when he's older.
I think
Frederick 'goes' well with
Hazel and
Iris, to some extent
Noelle, but I don't think it fits in quite so well with
Anouk and especially
Nyah. But I don't think sibsets have to 'go' together anyway.