Here's my experience w/ rainbow sharks and tetras:
I have had a rainbow shark for almost 4 years now (he was my first purchase after my tank cycled w/ zebra danios)... he's impervious to ich (2 outbreaks), and heater malfunctions (tank went up to 85 and was like that for almost an entire day). I really like him, he's my favorite fish, his name is Darth Maul (hubby named him). He also spends most of his time in a cave, coming out for food and to 'stretch his fins' every so often during the day.
I got a school of 10 neon tetras about a month after we got Darth... and lost all of them to a disease that affects either just neons or tetras in general (can't remember the name)...
I then bought another school of 10 neons this past Thanksgiving. They were small ones - 3/4" long maybe? Over the course of a week they all disappeared, no body parts or bones to be found anywhere in the tank. At the time the only other inhabitants of the tank were Darth, a common pleco and a betta....
Then I bought a school of 8 cardinal tetras in February. I only have 2 left, and again, no body parts or bones to be found!!! The only culprit I can think of is Darth.
I had 3 serpae tetras about a year and a half ago, they lived for just over a year and then my tank got ich and they didn't make it. But Darth never seemed to bother them at all. And he didn't bother my (penguin? black and silver w/ a red glowing stripe) tetras, who also succumbed to the ich.
Good luck!!!!
~Tara