Would: "Znam tu wszyscy". Also be a correct sentence in Polish and if so, would it mean the same?
No, it's Accusative for masculine personal (the sentence implies 'każdego człowieka' in a way).
It's masculine personal/animal accusative, which is of the same spelling as the genitive.
I'm guessing that if "everyone here" is female, then that would be "Znam tu każdą" or "Znam tu wszystkich" (as, according to Wiktionary, wszystkich can be either masculine or feminine). Can anyone confirm if this is correct?
Is każdy only used for persons? Than what would be the meaning of I know everything? And is there any difference in meaning of znam wszystkich/każdego?
No, "każdy" simply means "every"/"each", so let's say in a cartoon about dogs a dog coming into a room full of other dogs could possibly say "Znam tu każdego", because the context makes it clear that was is meant is "every dog", or if you prefer, "everyone".
Well, "I know everything" is completely different and means "Wiem wszystko". The verb changes because "to know" here is about having some knowledge, and not 'knowing someone'.
No, at least in this sentence, "wszystkich/każdego" mean basically the same.