What you describe is a mixture of species from different areas and with different requirements (which you could never offer in a single communitary tank, so you were sacrificing somewhere). It might have looked to you like everything worked well, but i very seriously doubt everything did. For one thing, a three year old Xenopus laevis could eat the newt and the bettas without a problem...and they would definitely try. Apart from that, Xenopus are known to be asymptomatic carriers of quitrid, and frequency of infection in captive stock is rather high.
The other three species are also incompatible between themselves, but the main problem were the Xenopus.
Most caudates species actively inhabit only fishless water bodies, rejecting those that contain them. They can detect their pressence chemically even if they can´t see them. Depending on the species of fish that you force unto your newts, you can have serious trouble. Small, completely unaggressive species like white cloud minnows or guppies, can be ok in small numbers and as long as the newts have plenty of hiding possibilities, but that´s about it. Bettas are definitely not ok and will cause at a minimum, stress. This can drive the newts to abandon the water.
I know you said you´ve already heard about this kind of arguments from this site, and it´s your choice if you want to ignore them. This was mostly for other people who might be reading.
