Threadfins prefer being in a larger group -- five or more -- and they need tankmates that are sedate. Also, if there is any question on whether your other fish choice would nip fins, do not combine them. I speak from experience here after my threadfins were decimated within hours. A terribly sad and costly mistake on my part.
I think khuli loaches prefer company, too? I can't recall. Plecos make lots of poop. All over. You'll want to be prepare for that.
I've always read how a tank looks better with a larger group of one type of fish, rather than several smaller schools, and mostly I agree. That's a matter of personal preference overall though.
A lot of tanks I've seen seem to push the stocking guidelines, and goodness knows I've done it myself -- but I have very good filtration, heavy plant load, and do regular water changes. I also don't stock to the point the fish look crowded in the tank. So you can be "flexible," as long as you're careful about your upkeep and stocking -- ten goldfish in a 15 gallon tank would be a disgusting swamp before too long, but ten tetras, say, would be just fine.