I used to keep a red tailed black shark with tetras and he was fine. He had his bit of territory and would do a bit of chasing when anyone else came near, but he never attacked them, he was a pretty sociable community fish really.
However I know they have a reputation for being aggressive, and it may depend on tank size, whether there are enough hiding places to establish territory, and on the individual fish - perhaps I just had one with a good nature!
My Red Tail is fine most of the time, he has his moods normally around meal times if ever. He sometimes chases the tetras, but it's only in a 'dog after the sparrows' way, he scares them, never nips.
Sometimes he scuffles a cory or two away from his cave, but there's never any harm done.
They like to have a space to call their own, so plenty of hidey holes is a must.
Mine hid away for ages, then when I got my betta the two became best mates and he's been out and about ever since!
They are very characterful, mine enjoys swimming upside down and 'walking' on his face! Don't worry, it's not a balance disorder, I think he's just eccentric!
I currently keep a Red Finned Shark in a tank with WCMM, Dianios and various Tetras, previously I had a Red Tailed Shark and that worked fine aswell. They are generally aggressive to fish of the same species or to fish who look similar to them as they are not the brightest creatures and get easily confused between friend and enemy.
my red tailed black shark is a bit of a sod......i call him the git. he killed my first two silver sharks......will never have them in same tank again. he was fine when i had tetras though...pretty much left them alone.