It is a decent book, but I think Old Man is right—maybe the book was referring to the construction of a knotted, tapered leader. Mixing different brands (and especially mixing different types like mono and fluoro) might cause a problem seating knots occasionally---that being said, I mix different brands of mono all the time. The brands of backing, fly line and leader can all be different, and usually are from what I've seen.
As far as your six weight rod goes, there are a lot of different choices. I’d go with a 6 weight line to match your rod line rating in a weight forward, floater. It would be designated something like “6 wt WF-F” .
As far as specific choices for an inexpensive line, Scientific Anglers Air Cel (about 25 bucks) would be a good one, with more options a bit higher (Rio Mainstream about 35 and Sci Anglers Air Cel Supreme 2 at 38 bucks).
Inexpensive house brands can be good, but it’s a bit of a **** shoot. Close out or over stock sites like Sierra Trading Post can also be a source of bargains, but look for name brands like Rio or Scientific Anglers.
You may also need backing for the fly line. 20lb test Dacron or Micron backing (any brand). A 100 yd spool will usually run around 100 bucks and should be more than enough for your reel. (There's probably some info that will list backing capacity for your reel with the packaging. It should probably be anywhere from 50-80 yds. If you buy this from a fly shop, take your reel along and have them rig it for you. They’ll put the right amount of backing on, and will be able to do the knot thing, and attach a leader to the fly line for you.
If you want to do it yourself, and are not sure of the knots to use, go to Animated Knots by Grog
Backing to reel- arbor knot
Backing to fly line- Albright knot.
Fly line to leader- Nail knot
Tippet to leader- Double Surgeons knot
Hope this helps.