The tag came out of this discussion. My thought when I made it was that community tanks are one of the basic setups in fishkeeping, the same way a reef tank would be, and they have their own set of questions and problems. It's not just about compatibility and behavior: there are also questions about water chemistry and quality, medical treatments that some animals in the system would be sensitive to, diet and feeding techniques, lighting schedules, etc. that are all specific to this type of system. So your examples would, in theory, be compatibility or behavior questions that deal with a community-aquarium. (I just don't think anyone has asked any of the other kinds yet.)
Let's say, for example, two people have a question about treating tiger barbs that have come down with ich. One person just has the barbs, and the other has a few (still healthy) danios and clown loaches as well. These two questions have different answers, because clown loaches are sensitive to many common ich medications: a good answer to the first question could kill some of the animals involved with the second.
You might say, reasonably, that the second question should also be tagged loach. The problem we'd run into is that even small tanks often have too many species in them to tag each one, and an aquarist may not necessarily know which, if any, are particularly relevant to the question. Or there may just be too many relevant species to tag. Or someone else in a similar situation might be looking through existing questions, but they have e.g. freshwater stingrays instead of clown loaches, and those just haven't been specifically tagged yet.
So the basic problem here is distinguishing between questions where having a single-species vs. multi-species tank is critical to the answer and/or affects who will be interested in the question in the future. That's what community-aquarium is meant to do.