Fashion rules are meant to be broken. However, in order to break the rules and be stylish at the same time, you need to know the rules. You shouldn't be breaking more than one rule at any given time. However, if you are on point with your style, you can make a fashion statement by purposely breaking a rule to your liking. It shows personality. If you are careful, even if you are breaking a style rule, it shows style. Like I said before, don't break more than one rule at once.
In order to get to know the "rules", I suggest reading Glenn O'Brien's "How to Be a Man." It is not really about how to be a man. It is very funny, gives social commentary, and most importantly teaches men about style. Also, you can read magazines like GQ, Details, Esquire, and more. GQ is the best magazine that I mentioned by far. Start off with that. Especially pay attention to, and study, the bi-annual "What to Wear Now" editions.
The "rule" with flip-flops and sandals is, unless you are actually going to the beach or to a pool, or going outside first thing in the morning to pick up the newspaper, gentlemen don't ever wear flip-flops or sandals. But by all means, do your thing. If you are wearing basics, like slim cut jeans with a t-shirt, or shorts and a t-shirt, go ahead and wear flip-flops or sandals. Just don't wear them with chinos.
But this is America. Honestly, do whatever you want to do. But I think that my advice might help.