Are there rules for combining generic variant magic items?

For example: Adamantine armor is an uncommon generic armor variant. +1 armor is a rare variant. Is there such a thing as +1 adamantine plate armor? What’s its rarity?

Does the situation change when one of the item "bases" is a generic variant, but the other isn’t? For example, javelin of lightning is a specific uncommon magic weapon. +1 is a generic uncommon modifier to weapons. What about a +1 javelin of lightning?