Depends. You could always go over the top but I've seen a few tanks that are totally overstocked and the fish tend to suffer. End up smaller etc from being stunted.
Convicts and kribs would both work. T bars and keyholes would probably also work.
You could try filling the tank with livebearers? They'd be happier in the low temps than the other types of fish. I can give you some free guppies and platies if you are keen
There is probably hundreds of threads on fish shipping, but is there anywhere around howick/botany that ships fish?
I have to drive to onehunga or something from mine to ship fish. Driving in rush hour traffic is such a hassle.
You can get fancies that droop into VT
But like others say. Watch out for finrot. I do tend to agree with keeping it separate though. The coke bottle method works well. Guppies and fighters can have issues and if the fighter is already weak the guppies may just make it worse