Hello
I am new to tying and I bought some hackle and it is for size 8-2 and I tie on hooks 12-18. Can I trim the hackle tips to the proper lengths or would that harm the fly.
In short, it looks terrible if you do! Takes the fine ends out.
I think the fish are a lot less critical than we are.
Jeff
Will it affect the fishing at all? Ya I know it looks terrable but will it matter if the hackle is to long?
cdpaul, It kinda depends on if your tying dry flies or wet flies. Wet flies, not much of a problem really. Dry flies, it will affect the footprint that the fish see from underwater, and it might also affect how the fly floats. You might get by with an 8 hackle on a 12 hook.
Next time your looking for hackle, visit a sponsor at www.conranch.com . Look at what Denny has to offer, then spend the money and call Denny. Let him know what your tying and the sizes, he will make sure you get the hackle you need to meet your tying needs. The crew at Conranch, Denny, Liz and Ken are all stand up folks that only sell to fly tiers, no middle man.
Wouldn’t ya just know it? We get an umpire on here and he asks if the fish can see that the feathers are low and outside. Unlike umpires, fish ain’t blind! Of course they can see that!
Now, to beg your forgiveness please, but I just could not resist it. Yes, the feathers are too long, cutting will just make the ends of them stiff and pierce the surface. Keep them until someday you might have the correct use for them. Getting the right size is very important, especially at your stage of the tying game. You will not have to spend a lot to do it either. Call Denny. You will be glad you did. 
cdpaul what color is the hackle. Is it neck or saddle? might trade ya for what ya need.