Yes, absolutely; it's amazing how most of our standard aquarium species can adapt, thrive and breed in a wide range of pH, hardness/softness, temperatures, etc...and yet it seems to be a trend now to fret about flow rate as if only a very specific amount of water movement will provide comfort to the fish. In a natural situation it would be easy to find 2, 3 or more species of fish found living side by side in a stretch of water, but whose feeding strategies keep them in completely different rates of flow. A pelagic species or a bottom feeder in open areas would be exposed to the highest rates of flow, but other species living an ambush predator lifestyle would spend the vast bulk of their time in a slack water micro-environment in the lee of a rock or other obstruction, from which they would periodically launch attacks on nearby prey items.
Seriously thinking that both of those types of fish could be housed in their "preferred" flow rate within the confines of a typical aquarium is delusional. The entire aquarium in most cases is smaller than many of those slack water zones, and yet we have keepers thinking that a few cubic feet of space can be used to replicate the various areas of an entire river. It's just not realistic.
Before we decide that a given fish species is "happier" in a social group or as a single fish...consider that fish, like all species of wildlife, are concerned about not only their own survival, but also to at least the same degree they are driven by the reproductive imperative. The desire to breed and perpetuate not the species, but rather their own bloodline within the species, is being frustrated whenever we keep a single fish. We do it because their aggressive tendencies become inconvenient for us; we can't keep a pair together because they may breed, causing problems for other tankmates, so we keep a handful of singletons together to minimize their aggressions. It makes it easier for us to crowd more species and individuals into our tanks...but think of the frustration we create for them! If you want something obscure about which to worry...there's an interesting target.
