Over the past two decades I have kept 3 undulates long term. In a 125 I had an undulate trigger (undulated in some books, still not sure which is correct) with a Picasso, lion fish, porcupine puffer, snow flake moray and (when it was legal) a leopard shark. All was fine until I added a blue spotted stingray. I my triggers in qt for a week while the stingray settled in. When I returned the triggers all seemed to be fine. Then next morning the blue spotted stingray was just a stingray. The undulate trigger had eaten each and every blue spot off the stingray. I returned the stingray and never had another problem in that tank.
In other tanks I successfully kept undulates with fish such as big eye squirrel fish, lipstick tangs and dog face puffers. The one thing I always attributed to my success was giving each of the fish (that use them) a hiding place with two openings, plus one or two extra hiding places that are empty. With this type of rock work, everyone has a territory. If one fish gets rambunctious all it can do is chase the other fish in and out of a few hiding places and will soon grow bored of the chase. Having the empty hiding places will provide for territorial disputes with out leaving a fish homeless. Always have an undulate's tank mates be a little larger and the undulate should always be added last. Since you already have the undulate trigger in qt I would leave him there as you stock your tank. Later on after your tank is stocked and the undulate is in, you may want to add another fish. To do this, simply put the undulate back in the qt and leave him there for a week while the new fish adjusts.
Someone previously posted that you could stock two tanks identically and could get two very different results. This is an absolute, especially with the infamously aggressive members of the trigger family. You should only add fish when you have a day to sit back and baby-sit the tank. The first night is also important to observe as the behavior of some fish changes drastically in the dark. With that being said, when I took these steps I always had a peaceful aggressive tank.