I would make sure both ammonia and nitrite levels are zero before adding more fish.
I've seen, black widows, zebra danios and fantails kept together with no problems.
Black widows would be better in a school so atleast four to six like you said. There are however gold or gold speckled black widows which could add more colour without adding a new species of fish.
Fantails can grow huge and produce lots of mess so liimit the numbers in a 50 L / 13 US gal tank.
There are so many cold water fish you could keep including white cloud mountain minows (gold or normal), leopard or spotted live bearers, golden medakas or even the black paradise fish (M. concolor) which is less aggresive than the "normal" paradise fish but aggressive when spawning, so maybe just one.
Bengal danios, rosy barbs and golden barbs (both rather peaceful barbs) are nice but grow rather large, around 10-15cm but a fantail can get this size easily.
I think without a heater cories wouldn't be a good idea in Adelaide as most cories like it above 22 C / 77 F.
Just a few things that came to mind...
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