Gorgeous fish, but less trustwothy even than puffers. If I had to co-house something, it would likely be a totally nocturnal algae-eating catfish - with a big driftwood tangle for refuge.
Species only tank is my opinion also (they may accept otos/etc but I err on the side of peaceful caution)
I wrote this a long while ago after doing a bunch of research on these for another site:
"The Bucktooth tetra, is a very aggressive fish that must not be kept with other fish, making them suitable solely for species-only aquariums. They have been known to attack even full grown oscars. Due to their aggressive behavior, this species must be kept in groups of at least 10-12. Otherwise, they have been known to kill each other. Groups of 15-20+ are recommended. The dissection of the stomachs of wild specimens furnished bellies full of partly digested scales from other fish; the Bucktooth tetra is notorious for eating the scales directly off of other species. In the aquarium, they will accept live worms, small live feeder fish, frozen foods and sometimes flake foods. The caudal fin of this species is yellow, while the other fins are reddish. These silvery fish have two very distinct black spots, one is at the base of the tail and the other is on their back, in front of the dorsal fin. Males tend to be thinner than the females and have longer dorsal and anal fins. Breeding is considered very difficult. Known to eat aquarium plants. Other scientific names for this fish that are sometimes used are Hystricodon paradoxus, Epicyrtus paradoxus and Epicyrtus exodon. Note: Due to a misinterpreted photograph in a published fishkeeping article, some people falsely believe that this species can be kept as tankmates for Piranhas. However, young Piranhas are likely to be attacked or devoured by Exodon paradoxus and any Bucktooth tetras that are placed with adult Piranhas will quickly become a food source."
My experience differs a bit. Now take in to consideration that my tank is planted and has a lot of breaks in the line of site. Its a 75g tank...with 13 exodons, 2 banded loaches, 2 butis butis, 1 nandus nandus, 7 otto cats, 5 maylantis cories, and 2 rubber lipped plecos and a senegal bichir for 3 months that i just moved to another tank. The only thing the exos have ever messed was my male convics, but he would flare at them...and one time they took a scale of the nandus, but then they realized that was a very bad idea. Since then i dont even see scales missing on any other fish (besides themselves), and from my observations unless it can fit in their mouths, or moves really quick and darty like they do, then mine dont pay them any attention. I believe that you should treat them like african cichlids....by "overstocking" them so that the aggresion spreads through out themselves more, to a point where they dont pay attention to any of the other fish. Also mine prefer bloodworms and flake to even small guppy feeders. Again this is just my experience in a little over 11 months of owning them. FWIW