Kyle Williams hasn't shown the ability to play a 2 gap NT to this point in his career and to try and use him there would be a total waste of his ability. He's no where near big enough and it would negate his quickness and explosion off the ball, which is one of his best traits. Like Dareus, he's best as a 3 or 1 technique DT. If the Bills played a 1 gap 3-4 like Denver then it might work out, but that's not Rex's system.
First of all, both Charles and Bryant are career backups. Secondly they can't play 5t, so unless you're going to play a 5-2 they can't be DEs. They are going to have to play on the edge some of the time in a true 3-4 defense. Thirdly, the fact that Dareus played DE in college doesn't mean he can do it in the NFL and even if maybe he can I highly doubt he will be as effective as he is playing DT.
If the Bills seriously want to play a 2-gap system then they really need all new DL except for maybe Dareus. So, what was the strength of the team 2 years ago must now be totally dismantled. This obviously implies that they should play mostly 1-gap schemes but they tried that this season and it bombed so we'll see.