The mono for the nail knot and first loop should be about .021" dia. or 25# test at least to carry the momentum from the line to the leader. Cheap mono is fine as it is not the weakest part of the system. The 'permanent' mono cannot be finer than the butt of your leader.
Also check out welding loops at the end of your fly line with a bit of heat-shrink tubing (later discarded). Here is the video. The melted part does not have to be as long as Peter makes it. You can also use a lighter, candle, or roll the tube across the top of a 60W bulb, but you have to be careful not to burn the line. If you do, just cut it off and try again, you only lose about 4" of line.
The slip-on braided loops are sometimes MORE flexible than either the line or the butt of the leader and this can lead to casting problems. As well, the braid retains water which sprays all over the place on the cast - possibly spooking fish.