Here is the
install guide for the 3M connectors. Looks pretty clear, but your mileage may vary.
The small connectors should be fine for 18 guage wire. (Are you sure you are not reading it as 19 strands inside 20 guage wire (that's a standard size)
Feel free to send your feedback to 3M -- 3M Customer Service at 800-225-5373. Thats another reason you got the part, they want to know about your experience(s).