Joe --
I think your examples of African puffers are not entirely indicative of pufferfish in general. The African species traded include species that are piscivorous. In other words, they aren't aggressive towards other fish, they are predatory on them. Totally different issue. Since pufferfish that eat fish will take either small fish whole or pieces of large fish (such as their fins or scales) housing them with anything except perhaps armoured catfish is very difficult.
The Asian species are different. Some are fish-eaters in part (e.g., Auriglobus spp, fin-and-scale eaters) or whole (e.g., T. suvatti). But others do not eat fish normally, such as figure-8s and Carinotetraodon spp. In some cases these fish may nip fins when hungry or out of curiosity, but they aren't "piscivores" in the strict sense. Mixing them with other species seems to vary on a bunch of factors that aren't reliably clear to me. I have found C. irrubesco to be entirely peaceful with tetras and glassfish, while others have found it to be a fin-nipper. One published expert on the group told me that among C. lorteti it varies depending on where they come from, the Thai specimens being fin-nippers but the Vietnamese specimens being altogether more safe with other fish and each other. So there may be some regional variation.
The South American species are different again. SAPs are not aggressive or territorial because they are schooling fish. I've seen them nip at things when hungry, particularly livebearers and Corydoras, but they are completely safe (in my experience) with fast-swimming tetras and robust catfish like plecs and Synos kept in a reasonably big aquarium.
The general rule -- freshwater pufferfish aren't community species -- is basically sound advice, but the reasons are complex and vary from species to species. In some instances, it may be possible to combine certain pufferfish with other species. Some people have done this and had success (myself include) while other people have tried and ended up seeing their other fish with nipped fins or worse. Obviously if the puffer is a predatory species, adding tankmates is very, very risky, but where the puffers feed on invertebrates rather than fish the risk is less clear-cut, to me at least.
Cheers, Neale
Currently I have representatives of every recognized species/subspecies except P. ansorgii, P. bichir bichir, and P. teugelsi (all of which I'm still working diligently to acquire).
