I think it means weighing up the [ body size / aggression / group size ] ratio.
Chains are smaller than zebras, but a little more robust. So they go well together - so long as they are in a moderate sized group.
I did have one dario. The only one I have seen in the LFSs in recent years. I put him in the tank and never saw him for a year. He had grown, but always looked a bit flustered, and never participated in the other loach activity. In the end, he withered and died. The [ right body size / not aggressive / only one ] ratio was definitely wrong.