I disagree about valentinis eating hermits. They eat much smaller inverterbrae hence its much smaller beak than alot of puffers. I have seen them kept in reef tanks quite happily as long as they are kept well fed with cockle and mysis etc.
A friend of mine keeps one in a 500L reef with various other fish from scooter blennys and yellow clown gobys up to a copper-band butterfly and a regal tang.
Its very difficult to say whether this would be the case with every valentini, but from what I have seen they are actually very peacful as long as they are kept nicely fed. However, keeping it in a 20G would obviously mean that you could have very few other fish with it as they have the potential to get to 4-5" so you wouldnt want to crowd it with other fish.
Ben