Is "Omnus nos eloquii mystici" correctly written to say "We are all sorcerers/enchanters"? I know there are different ways to say sorcerer or enchanter, but I really want to use "eloquii mystici". I just don't know if it's written correctly in plural.
Omnus is probably a typo: you want omnēs to mean "all". Nōs is the correct word for "us".
Now, ēloquiī literally means "of eloquence" or "of an utterance", while mysticī is an adjective meaning "mystical" or "relevant to secret rites". So the sentence currently means something like "we are all [the people] of mystical eloquence". A bit of a strange phrasing, but not incorrect.
Since you mention "in plural", however, I'm guessing these words don't actually mean quite what you think they mean—as it is, they're singular in your motto. If you clarify your intended meaning a bit, I might be able to help you more. (For a word for "sorcerers" or "enchanters", I would use the straightforward magī or the Greek-derived goētēs; most of the other Latin words for sorcerers are pretty negative, meaning things like "curse-maker".)