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I keep rummies with angels and have never had a problem. Of course, if the tetra was tiny and the angel was big, there is always a chance of a tetra salad. Some angels are more predatory and I have seen them eat platys and cories even though those species are generally considered angel-safe as well. Sorry about the expensive lunch.
Angel is pretty big. I think the rummy may have been dead or dying. All I know is I put six in and cut off the lights. When I cut them back on my angel fish had one sticking out of his mouth!