Well, I didn't vote to close that question before because I totally forgot that it had been asked before. I'm betting the other vote also is because it's a duplicate question. You're right that we don't really need two Q&As with the same content.
So what happens is the first person to ask a question gets to keep it, and everyone else gets told to refer to that question. Unless there's something wrong with it, like it became outdated, or the first question was written really badly.
The idea is that we don't want to end up with many different questions on "What fish get along with neon tetras", because then our answers would get spread out between the questions and someone who is looking for the answer has to look through each Q&A rather than just scrolling through a single set of answers. It's easier for us to keep a single question answered, than many duplicates
Someone who has spent more time with dish might be able to give better detailed guidelines, but really the only guidelines I can come up with that apply to all fish are: Big fish eat little fish, and Cichlids are jerks.
You can try to follow a chart that some one made (This is Petco's):
Petco's fish compatibility chart http://www.recipeapart.com/wp-content/uploads/2010/11/freshwater-fish-compatibility-chart.jpg
A lot of the problems with fish compatibility is that it's partly the personality of the fish, and the experience of the person who made the chart. Right off the bat, I can say that my experiences are vastly different than theirs, because I would never put barbs or danios with Angelfish and sharks don't go with other sharks.
Perhaps if there was a question that asked something like "Are there general guidelines for fish compatibility?" we could give some answers. But I'm inclined to say that it's more useful to have questions like "I have these fish, and I want to add this fish, is that okay?"
I'm still up in the air about questions asking for any fish that get along with a certain fish. It's a bit of a list question to me, but sometimes it seems to work okay. I think it all depends on how the question is asked, and how much detail they give us.
On a side note. I've been meaning to update the aquarium tag to say that questions should include details about the tank, including size and chemistry if possible, but the people that need reminded won't really read the tag wikis (as they're pretty much always new/one-time users). Unless we can have a pop-up when someone selects the aquarium/fish tag that asks them to include the information, all we can really do is ask them in a comment, and if they don't give us enough information we would close it as such.
Technically, the size of the tank doesn't matter when asking if fish get along together, but I ask it because I like to know not to suggest fish that are too big for their aquarium. I know that if I suggest a fish, even if I say it's too big, they'll cram it in anyways. So it's just better to pretend like that fish doesn't exist for them.
In that case, it's probably better for people to ask "Is X compatible with Y" questions, rather than "What is compatible with X" questions, because it's a more direct question. I don't need to worry about the size of their aquarium with that type of question, it's yes/no and a reason why.
So I'd have to say that personally, I think any question asking "What fish gets along with X" or "I have X what can I get" questions are too broad. We should really work on having people do some research on the fish first, pick some out and ask if they're compatible. That way we can have clear questions and not questions that are one sentence and an unrelated picture.
At the moment, I don't think it's become a terrible nuisance as we don't get those types of questions often, and I don't know how other people feel about it, but we may have to revisit Are "Recommend a fish questions off-topic? and come up with some rules on what questions fit into our site, and which ones don't promote good answers.