Re: After the draft...
The never draft 2 defenses rules should be taken with a grain of salt. It depends on your leagues scoring for defense and the size of the league. If it's a small league (8 or 10), generally yeah, don't draft 2 defenses. You can play the match up game in those leagues fairly easily.
12 team leagues depend on the scoring for defense. In standard scoring you can pretty much still do a 1 defense matchup mix all season long. In some custom leagues though defenses can get penalized heavily for allowing alot of points/yards, and if they don't get return yards added to them it doesn't create that buffer that allows a bad defense to stay on par with the good ones, on the flip side in those leagues defenses that hold opponents to few yards/points get monster points (Bears had a 60 point game in our league last season, good defensive outing usually net 25-30+, average teams 10-20, bad teams -5 to 10). In these leagues it is smart to try to land 2 defenses, it doesn't have to necessarily be an early reach for defenses, just two "draftable" defenses that you can play match-ups with between them.
The thing you have to remember about trying to play matchups against the weak offensive teams is this. The Strong defensive teams play the bad teams too, If your league has a lot of owners (14 and 16ers for example) or puts an emphasis on defensive scoring, it could be slim pickings when trying to play the waiver wire for a defense each weak. There have been times I've been stuck with a mediocre defense playing a good team because none of the defenses playing bad teams have been available.
Low owner count and/or standard scoring = 1 Defense/playing waiver wire good
High owner count and/or defensively favored scoring = 1 Defense/playing waiver wire bad to potentially catastrophic.