Eels, in general for the most part when in their young juvenile stages do often appeal to be more active then when in their adult stages. Myself, as far as mixing with a more aggressive F/O tank, you would need to allow as many hiding places as possible. The idea with puffers and eels, I always look to not house the two together, for one thing if or when the puffer fish were too take a bite out of your eel, it could cause some serious injury to which can cause the demise of your eel, and as for a triggerfish, I would select one that appears for less aggressive.
So you know now that eels while in their juvenile stages, do appear to be moor active, as I seen all too often through the eels keeping a great many different species, that when they grow, most take to their normal active behavior by hiding most of the day. But your idea on some such a mixer of fish species in a 125, im afraid to have to say that in my opinion, you will not be able to manage an adult angelfish, the tank is too small for this selection.
Also, if you did have a bit larger tank, and you did lets say gotten yourself any large angelfish and of the wrasse family, most always are selecting the Harlequin Tuskfish (Lienardella fasciata), this mixer in no matter how large a tank, can become a bad idea, for the adult angelfishes often attack the Tuskfish. It was like this seafood restaurant, they have a 1000 gal tank, a odd shape large tank to had so many angelfish with a tuskfish. But I told them that they should remove the tusk or the angelfishes to which they desire more to have, for the angels will some day attack the tuskfish. It was a few months later that I heard about their tuskfish died from injuries due to the attacks of the angelfishes in the same tank.
Also in what you said that your LFS has a number of the chainlink eels, in the same tank as a group, eels find safety in their numbers and are often active. But still they are eels and each of their survival will depend on its own will to survive, not on a group saying, all for one and one for all. Eels are often aggressive among their own when it comes to feeding, and this matters not even if you were to house a well known mated pair.
So when the chainlink do grow, it will lay around in one spot for hours on end, and in a reef system with zero aggressiveness, it could even be seen in the near open areas of the tank. But you are looking to house this eel with aggressive fish, so it will require the ability to hide it self in total darkness, lairs as they be. Also, if you get this eel or any eel, during its young juvenile years, it is most surely an easy target for aggressive tank mates. A QT however would assist you well until your eel gains some growth before adding to the main tank.
Buddy