nice to mix with cardinals, IMO
I had 6 serpae tetras with about 30 cardinals, some cories, some kuhlies and a few other fish. The serpaes tried to chase the cardinals, but just succeded in making them school a bit. Cardinals wre way to fast to catch and they all look alike so it was hard to chase any one fish. The serpaes did tend to intimidate the cories sometmes, but it is a big tank and the cories could get away, as many cories as serpaes. The serpaes tried to stare down the kuhlies but they couldn't care less.
The serpaes are the most interesting fish in the tank, the males are constantly "sparring" which is lovely to watch, the females tend to stake out a corner of the tank and tease a male into the bushes back there occasionally. Be sure to have a nice open area for the sparring matches, it is like watching karate or kickboxing, but they never make contact, they just rush and twist away.
When I had an accident with lack of dechlorination, I was slow to replace the cardinals, but quick to replace the serpaes.