I've noticed a lot of otc mouthwashes/fluoride rinses contain citric and/or phosphoric acid. wouldn't those harm tooth enamel or is the amt too small?

Accentuates results: OTC mouthwashes do, in fact, contain some acids and their ph level tends to be acidic because of that. The effect on teeth, damage to the teeth is minimal, but the effect on the surface of a tooth allows fluoride to integrate with the enamel of the tooth and strengthens it. So an acid environment does not necessarily cause problems if it is needed.