They can be kept in a community tank. I had mine in a thriving community of very active fish and they used to be all over the place hunting for food and chasing each other around.
However, I agree that they really do best in at least a group of six individuals. Someone on another forum said he had noticed a massive difference in behaviour from when he went from six to around ten individuals.
Also going by your signature you need more emperor tetras. One is definitely not enough and you really should have at least five-six. I believe you mentioned some problems with certain numbers in a previous thread, but honestly if you can't meet the schooling requirements of these fish you probably should stick with fish that do better either by themselves or in a trio.