Oops, I forgot to edit my post all the way so it would be apparent what the B-tension screw is. If you look at the rear of an XT or LX derailleur, there are the two limit screws, and above them there is that third screw that I was saying to be careful of. That's the B-tension screw, and its job is to boost the strength of the spring inside the upper pivot if necessary to prevent the bumpity-bump problem.
Go with the H and L screws as they come right out of the box at first, that is generally a good place to start. Then go through the procedure described, to fine-tune them.
Putting the cable adjusters most of the way in is a good plan, since cables stretch and you may need the range of adjustment later to take out the slack.
When pulling the cable taut, you can just pull firmly by hand, or grab it with pliers and apply more force. It'll all come out in the wash when you do the next steps to dial in the indexing.
With a chain and cassette from different bikes, it could be that you'll have chain skip if they're at different stages of wear. Just like you wouldn't re-use your old pistons after having a car's engine block bored, you can't expect worn parts to necessarily mesh well with parts at a different stage of wear (newer or older). There's a sweet spot, hopefully your parts are within it.
