So...I hate to be the bearer of bad news but I'm going to have to say it
The Colombian Sharks are not only brackish (and do best in full marine when adults) but are also extremely predatory (just recently someone on a different forum posted about their tiger barbs being eaten and I had one eat TWO full-grown rainbow fish). In addition, they will outgrow the 40-gallon within months (provided you have babies...if not, they've already outgrown it). They grow quickly in the first year or two and then growth tapers off slowly and they seem to gain about an inch a year after that.
Colombian Sharks also have venomous spines so I urge you to be especially cautious. They should never be netted nor should you handle them without gloves. Watch them closely when you maintain the tank, they get spooked easily and may crash into you by accident.
They're also jumpers. The tank needs to be completely covered, there can't even be a gap where the filter goes otherwise they WILL escape.
I love the species (and have three myself!) but they're definitely bad fish for beginners. My advice would be to take them back to the store. Otherwise be prepared to set them up in their own brackish, well-filtered tank of at least 50-gallons and be prepared to upgrade them to a 75-gallon or larger within a year. If that's the plan then be aware that you need MARINE (not "aquarium salt") salt and no, you can't add it to the current tank (the others can't tolerate it).
The pleco is probably a Common Pleco and believe it or not, he'll outgrow your set-up. They get HUGE, Google it. You'd be better off swapping him for a smaller, more peaceful species. A Bristlenose Pleco would be perfect
The Gouramis and Angelfish should be OK together. Both species enjoy having acidic to neutral tank conditions and lots of plants to hide in (live plants being ideal). The Tiger Barbs are hit or miss, they can get nippy with slow-moving fish like the aforementioned species. Just keep an eye on them.
The "dinosaur eel" isn't an eel but actually a Bichir (that may be useful to know when looking up info). They'll swallow any fish they can fit in their mouths so make sure none of your fish are that small. Your tank isn't really ideal for a Bichir and I'd consider returning him and getting some Kuhli Loaches instead. They have that same eel look but are hardier, easier to feed (Bichirs are all wild-caught and are generally very stressed and prone to hunger strikes), and do better in a new aquarium. Bichirs do best when introduced to a mature tank that's been running for a while. If you keep him, provide lots of live plants. Honestly, though, your tank sounds like it's more tall than shallow which isn't great for an air-breathing bottom-dwelling fish like the Bichir. Long, shallow tanks are better for them.
Whatever you decide to do, act quickly. You're right, the Colombian Sharks are not acting normal.
EDIT: I also want to add that the Bichir is an escape artist. They may not be eels but they escape just as easily as eels! ALL openings need to be completely covered.