It depends, I've had Buenos Aires tetras in with small central american cichlids (convicts, firemouth, herotilapia) and they didn't get eaten, but the larger and more aggressive the fish the more likely they will be eaten.
Smaller fast moving fish, most people use them to make a timid cichlid more comfortable, and alsi liven up the tank. Giant danios and Buenos Aires Tetras are commonly used, as are rainbowfish for bigger tanks.
I douby that the OP's oscar won't be making lunch of an adult male red hump...
I know the question wasn't for me, but there are some parasites, like intestinal worms that take longer to show than typical stuff like ich, velvet, etc.
If headcougar were to remove the sharks, the rest of the stock list (I'm not sure about the gobies, I'll admit) doesn't need to be kept in brackish water, so you might as well keep it as freshwater.
the only thing that bothers me is if I go with the caudopunks instead of julies, I fear they will compete too much with the shelldweller I end up choosing.
I've converted what was my planted tank into what will soon be a Tanganyikan tank, and I have the tank basically with a pile of rocks and one side and the other to be for placing shells. I've had some input already but here are my final ideas on stock list for the tank, and I basically wanted to...
you could theoretically add a small cichlid tankmate like a red hump eartheater or something along those lines, but you would really need to beef up the filtration and water changes significantly.
I remember being here back in the day (under my old screen name 'ruben') and posted here very often, and was aeven a mod in the brackish forum, but I pretty much disappeared for a few years until recently.