This "rule" is seldom accurate. It can be somewhat useful to guide beginners with small fish (tetras, corys, guppies) but if you think about it--12 1-inch tetras and one 12-inch oscar are two very different things with respect to their impact on the bio load.
The reason taller tanks are sometimes more difficult has to do with surface area. The volume in a 10g hex and 10g normal may be the same, but the surface area is very different; and it is at the surface that the gaseous exchange takes place--oxygen enters the water and CO2 is driven off. Then there is the natural behaviour of the fish. Fish that like to swim need length, not height; zebra danios for instance are active swimmers and should not be in a tall tank but always the longest possible to provide swimming distance. Quiet sedate fish tend to do well in taller tanks, like angels (though not in a 10g, this is just as an example of fish behaviours).
I always suggest that building up a community of fish requires fish that are compatible, and that has several aspects all equally important. They should have identical or near-identical requirements respecting water parameters (pH, hardness, temperature), environment (plants, rock, wood, type of substrate), filtration (some need more water movement, others need little or none), light--and I haven't even mentioned behaviours yet. Iamntbatman has a good article on this as a "sticky" at the head of this section, for ease here is the direct link: http://www.tropicalfishkeeping.com/f...tocking-38626/
It's all in there.