As I mentioned before, there is no trigger that belongs in a tank smaller than 75 gallons. The undulate is one of the smaller species, but they are active swimmers and shouldn't be housed is a small tank. I also don't know where this grouper advice is coming from, the vtail is a smal species but that is relative consider most groupers need 200-300 gallons. I can find no website that advises a tank smaller than 100 gallons tank for the vtail. Plus IMO the online retailers lean towards the small side for recommendations so they sell more fish.
As far as a wrasse, you have a TON of options. Not all of them are really that aggressive, some are even reef safe. Most of the more aggressive ones are too large for a 55, but there are plenty you can choose from that are small enough for your tank. You should also be aware they are often known to jump.