Learn Spanish (Latin America): Admite a todos, menos a ese hombre.

When using an exclusion word like menos, excepto, or salvo, you still need to use the personal a if the excluded noun requires it. Since ese hombre is a specific person receiving the action, it needs the personal a, perfectly mirroring the a todos in the first half of the sentence.

Could I use excepto or salvo instead of menos here?

Yes, they are completely interchangeable in this context. You could say excepto a ese hombre or salvo a ese hombre and it would mean exactly the same thing.

Is admite a command here, or does it mean "he/she admits"?

Based on the English translation, admite is an affirmative command for the informal tú (you). The affirmative tú command form is spelled exactly like the normal present tense for él/ella/usted, which is why you rely on context to tell them apart.

Why do we say a todos instead of just todos?