Learn Italian: Noi sappiamo che ciascuno ha un talento. - We know that each one has a talent.

Yes, they are mostly interchangeable! Both take a singular verb and mean 'each one' or 'everyone'. Ciascuno emphasizes the individuals within a group a little more, while ognuno is slightly broader.

Since 'everyone' implies multiple people, why is the verb ha in the singular?

Just like 'each one' or 'everyone' in English, ciascuno is grammatically singular. Even when referring to a whole group of people, you must always pair it with a third-person singular verb. This is why we use ha and not hanno.

Can ciascuno ever be feminine or plural?

It can be feminine (ciascuna) if you are referring specifically to a group of women. However, for a mixed group or a general statement about humanity like this one, you default to the masculine ciascuno. It never takes a plural form.

Why does this sentence use sappiamo instead of conosciamo for 'we know'?

Sapere is used for knowing facts or knowing that something is true, which is why it pairs directly with che (that) here. Conoscere is used when you are familiar or acquainted with a person, place, or object.