I have never bought the 100s because I don't tie just one size. I use necks because I like to use the spade feathers for tailing and tips for wings. The 100s might be a good idea, but it just really depends on what you want to get from the purchase. I would never by hackle that I couldn't take out of the bag and look at, so the local shop or a show is the only place I buy.
On clipping hackle, don't do it. At least not if you can avoid it. The fly will float fine for the most part, but it will look different from below. The blunter ends of the clipped hackle will make a larger dimple on the surface and may also break through the surface tension more readily. It also will just not look good to the eye and you will not be happy with the results. If you are desperate for hackle, consider parachute patterns where the size is less important, or comparduns,CDC wings, etc.