A picture of the Holly in question would help and a rough description of the results your looking for, that way some folks can get back to you with their views and maybe experience of doing the same ????
Without knowing why you would want to JUST shear the top off the Holly, and the results you want to achieve, then the answer is YES, you can just cut the top off any plant BUT do you want to reduce the HIGHT, do you want to prune into a shape, do you NEED to actually shear the top or would pruning the sides help you get more from the plant, or would digging it up and moving it be a better result and plant another more suitable shrub in it's place.????
Can you get back with more info and a picture would be even more helpful.
Because it is growing sparse at the top and 2 of them are growing at a different rate. I want them all even as they are against my light colored house as an edge. Yes they are narrow. Im just not sure if it will the upright branches will double and grow awkwardly.
I don't think your plant will grow awkwardly after shearing the top flat. As with most plants, if you cut off a stem, new stems will grow from the one or two leaf sprouts next in line. That plant is a very commonly sheared plant so you should do fine to give it a little hair cut to make the one the same size as the other.