You can use either "to" or "być (e.g jest)" for "noun is noun" sentences, I am pretty sure.
As far as I know, the only hard rule against using "to" is that it is never used to link pronouns. And "to" uses the same noun case on both sides, whereas "być" needs a different case.
Well "są" is plural, it's like saying "are". I've heard that it sounds weird to natives when you use "to" with people (like Anna = Woman). I'm not sure though, I'm not a native, or good at this :')