@Mamabear1921
Different question. Thank you so much for your comment.
It is kind of answering the question though.
You asked 'can you get away with' - what you really mean is, will this short form 'stick' and will other people call her it? Because of course being the parents you can 'get away' with whatever you like! You can call her Hallie, introduce her as Hallie - all fine. Yes it would 'stick' for all those people in her early years who YOU introduce her to and who know you, the parents.
Will it stick - not necessarily.
If SHE likes being Hallie and keeps that short form and introduces herself as it - yes. If not - no. She will be Harriet, Harry, Hattie - it'll be her choice.
So in that case you won't have 'got away' with it.
So the real nub of all discussions like this is - you can choose any short form you like and it can be as tangential as you please.
However, it will only be in your power for as long as your child is very small. So if you choose a short form that isn't the 'natural' one, you are indeed risking your choice of everyday name disappearing. And if you actually dislike the natural short form (like a Hattie or a Harry) then you're playing with fire as your child might prefer that for themselves and you'll have no say. That's the real thing to consider.
So I'd say. If you love Hallie, but also like Hattie and Harry and wouldn't mind if she ended up as that -go for it.
If you hate those short forms, beware, because nicknames and short forms evolve beyond your control as soon as they are out of reception.
Friends have an Alexander. Adamant he wouldn't be Alex as they don't like it. He is Alex and has been since he decided to be, around age 6.