I used a white version on my build and didn`t have an issue but you could tape it on the inside just to be safe
And you don`t have to taper it if it restricts your movements, it looks fine as is, from the pic it looked like there was some room for it which would help in giving that sleek "movie-look", however doing so wouldn`t make the top to flare out, just make sure it`s securely taped in the back, at the very top of the thigh keeping it together as you reduce the eventual gap at the knee. AND...You might want to wait in doing any final cutting until you got your shins taped up to see how it looks together---it`s all about the look Which brings me to mobility well...it sucks..plain and simple someone wrote somwhere on this forum that "all the rebels had to do to prevent the stormtroopers to charge their secret base was to ad a step or two" If you want/need to you can cut some out of the back of the tighs/shins like these troopers did (a bit hard to see)
Personally I want to minimize the black showing so I didn`t cut any and this is how I walk (sorry if I clutter your build with all of my stuff)