You're about a step a head of me, again! :) Its great you've got that. A good crimper made life much easier when I replaced a whole bunch of wires and wrap on my jeep last winter.
I've done stuff like that too (take pieces from old harnesses). I just didn't want to post that on the 'net. You're the best judge of condition. Nothing wrong with old - if its in good condition.
Some people hate solder joints, but they have their place. I'm pretty sure I redid a wire connection to the fusible link on my wagoneer with a solder joint. Unlike our mopars, there's no connector between the link and the feed wire. Maybe I should have put one it, but at the time I didn't know enough about how it worked - I just wanted a good connection.
p.s. I thought this guy had some good tips on soldering splices.
Connection options, splicing wires and how to do it right, and WRONG!