How do carrots added to butternut squash/leek soup taste? Is that ok?

I often make a similar soup - I think everything you mention is made to go together!

I agree it would add some sweetness which you can cut by putting a bit of lemon juice.
Maybe garish with a dollop of sour cream to also cut the sweetness, and a sprig of something green for color.

I would imagine that they would add a wonderfully sweet taste to the soup, and also add to the depth of the color. Go for it!