Ropefish are peaceful as in they don't fight, but they still are predatory and will still eat fish that will fit in their mouth. An adult one will eat -2" torpedo shaped fish such as neons and rummy nose tetras. Taller bodied fish like Black phantom tetras are safe though. They don't have the greatest eye sight although their sense of smell is pretty good, so they may not wipe out all the tetras that quickly.
Dragon gobies are nearly blind, even then their sense of smell isn't that great, so for these guys an owner would need to check that they are still managing to get enough food. Even though they look crazy, they are peaceful and your tetras should be safe. One big thing to know is that they get huge, up to 2 feet in length, and yes they really can get that long in aquariums. I think there is even a video of a lady that has two, 2 foot long dragon/violet gobies in a 55, definitely still not enough room. And I believe these guys do best in brackish tanks (slight salt/salinity). Not sure of your tank dimensions, but probably still too small for a full grown dragon goby.
Bichirs get big and even thicker bodied with a bigger mouth so those will definitely gobble up your fish.
Not really prehistoric-dragon looking fish, but still worm/eel body-like, are weather/dojo loaches, kuhli loaches, many other different loaches actually (with varying sizes and temperaments).
Kuhlis are extremely peaceful, but do stay small.
Dojo/weather loaches get up to 10 inches and pretty girthy. Still peaceful, but I have seen videos of them being pretty bulldozer-ish during feeding. Not aggressive, but it looks like they just plow into whatever is in their way, but I don't think they would really damage plants or uproot most.
Tons of other eel-like body loaches, but not really prehistoric dragon looking.
Gobies and darter fish look cool.
Other than those type of body, you could look into exotic plecostomus. Some very cool looking species. Varying size and diets so do your research. But all (even carnivores) would be compatible with your tetras. Not technically ancient, but they are very old. Some can look like dragons. You could even look at green dragon Bristlenoses (pretend they are horns haha). Even look at the long fin varieties to pretend they are dragon wings.
Other loaches like Homaloptera can look like mini dragons, but these hillstream loaches should be in cooler water with high flow and high levels of dissolved oxygen.
Some cool looking prehistoric creatures you can look into are Triops.
There are more prehistoric "dragon" looking fish, but they are all even bigger.
Maybe a elongated crowntail male betta or even a dragonscale betta?
Some wild betta species look cool.
Don't fit the criteria, but cool looking odd balls, there are smaller species of Knifefish and spiny eels, but that may be pushing it. Mormyrid are cool odd balls and some might work with tetras.
So nope, the aforementioned ropefish, bichirs, and dragon gobies wouldn't be good choices.
