I've found that a razor saw is vital for these guys, mainly for the restic Corporation bodies, as its the only way to get a straight edge. I also found that cutting them in half below the belt or above the belt (so to speak) was better for the torso's/legs (as shown in the photo). The zombie/ghoul arms can be cut off easily enough but be careful to keep as much of the shoulder as possible, then it's a straight forward case of cutting off the bump on the corporation torso and then trimming the arm to fit and glue into place. If you have a glue activator it'll help, as they require superglue and the restic doesn't meet the plastic that well to begin with. You'll need lots of little blobs of green stuff to help position arms, torsos and heads together. Other than that they do require a bit of trimming on the plastic torsos as they have a natural bump to fit into the legs, so cutting these to form a straight edge I've found to be important, though it doesn't have to be that big as greenstuff can fill the gaps.
They do require work but they also look great when done and you can't really argue with the price. I tried to go for a 50:50 ratio between restic and plastic but found that a few just didn't fit right, so I used a combination of ghoul/zombie bits instead - either way they still blend in nicely with the unit once painted. I also find their size to be great when compared with Marines.