I use a ton of soft hackles... hundreds per year... I give away a lot of flies!
While guinea is one of my favorite materials (I literally have a BIG tote of guinea skins... 50??? No, more...) it would not be a favored soft hackle... many smaller birds have really nice smaller markings that really work well.
Up front, I admit it ain't the arrow, it's the Indian. If you like them and trust them you will fish them with confidence and they will produce! But consider quails, grouse, starling, and such for some finer markings that will really look good and IME&O fish really well.