Let's adress these factors one by one:
1. You think a true friend would be angry at first? Even if jealousy sets in later, they would definitely ALL be caught up in the moment of excitement for their best friend.
2. When it comes to flying, someone had brought up Rarity's ability to fly when she had enchanted wings. It could be just that, for all we know.
3. How is it too much or too soon? This is a chance to see something new happen, have a new focus, have new events we couldn't have before.
Why do you WANT her friends to get envious of her anyway?
Also, although she could be a "princess" (which I am thinking is their equivilent of being knighted in this world, at this point), who's to say that the other 5 haven't been given any rewards offscreen?
By the way, another thing I'm surprised most people haven't considered: So...Twilight's now a Princess. Doesn't a Princess usually need a royal court?