For this room I would go Egg Chair or Shell Chair.

My sofa is low back, will high back accent chair look weird with a low back sofa or people usually like this kind of contrast?

Leather won't "soften" your look. Let me find some alternatives for you.

While it is true that the leather will not soften the look, the curvature of the proposed chairs will soften the look.

BTW, when all the chair heights are at the same height (in your case low) it is very boring and so a change in height would actually be a good thing.

You have some nice chairs from which to choose, but, I would like to suggest you do not go the tan route…just a bit too blah, IMO.

Try finding a leather chair in this color, instead. Lighter and softer…?

Love to see some white on the sofa, as well.

Light Blue Diamond Decorative Pillow, 21"x21"x6.7" · More Info

YES. I love the idea of those tan (almost orange) leather chairs in the original post. It's the perfect transitional pop of color with your blue rug. I like the high wing back chair or the lower "Go Mad" style with the wood frame. I've included an image/link of an ottoman that if you could incorporate somewhere else, could really tie the room together!