If you want to break the rules, I’d say knowing them is not enough. You have to understand why they’re there in the first place.
Classic menswear has a lot of rules regarding fit, colour and pattern combinations, how the clothes relate to particular circumstances. They describe almost every aspect of the dress: from how a shoulder line should look, to what’s the garment position in the complicated hierarchy of formality.
The most important of those principles can be acquired quite easily, though. There’s ton of fit guides all over the internet. The pattern matching can be summarised like this: vary the sizes of similar patterns, keep the sizes of various pattern similar. The formality ladder also can be brought down to a few fules of thumb: the darker something is, smoother and more shiny, the more formal it usually is (with the exception of the white shirt). There, now we can set sail and navigate the sea of classic menswear.
Those rules and principles won’t replace a certain acquired sense of aesthetic, and having some experience with classic tailored clothes made of quality materials and designed with the right proportions and silhouette in mind. Only this, in addition to knowing the rules and understanding where they come from, allows one to create and refine one’s harmonious and consistent style. These rules found in books and on blogs will then become only a starting point. They can be bent or broken, with a particular effect in mind.
When it comes to fit – all those jacket and sleeve lengths, shoulder widths, and so on – those classics are safe bets. They’re the most universal and work well almost every time. There may be some variation though. Understanding how those things relate to how your body shape is perceived, may allow for deviating from the safest solutions. If you believe a shorter jacket, a bit higher buttoning stance or a bit wider shoulders simply make you look better – these will be all good ways to dress, even if not proper in the eyes of people educated on basic menswear guides.
The rest – pattern and colour matching, formality, and so on – this is way more subjective. A matter of taste, basically. Not that I’m satisfied with such a conclusion, so I wonder what makes some deviations from the canon pleasant to look at, and some not so much.
I think two things stand out. One: to not overdo it. Knitted tie with a nice, dark, formal suit? Sure, but in a muted colour, made of a slightly shiny silk – and as the only thing “wrong” with otherwise proper outfit. Two: the rest of the outfit must show you know what you’re doing. The fit must be spot on. It’s obvious, but every imperfection in how the clothing fits magnifies the “incorrect” stylistic choices. Next: the outfit shouldn’t look like it’s made of poor quality fabrics, and not at all well cut and tailored. I say “look like” for a reason, because taking proper care of one’s clothes may make them look good for quite a long time, even if they didn’t cost a fortune. I’m not the kind of person who thinks there’s no way to dress well other than Neapolitan or Savile Row bespoke. It’s just that a shiny polyester tie doesn’t exactly communicate the awareness of the aesthetics of the outfit.
To do this properly, you need training. Intuition when it comes to judging the outfit, and critical view of it, is shaped by experience and a little bit of effort. But the effects can be marvellous. Not only when it comes to rule breaking. Otherwise you can be dressed head to toe according to books by greatest style gurus, and still not be stylish.