I have four flow hives- and my brother and mother another one each. All of them are running off one brood box- except for my long hive which is around a brood and a half. there is absolutely no problem running a flow hive in Adelaide off of a single brood box. All our hives are wintering well- and in fact the bees are out almost every day bringing in loads of pollen. This year we also left our flow frames on over winter and so far this seems to have been no problem- though you do need to keep an eye on ventilation (coreflute in bottom slot and/or top ventilation of some sort).
This season I plan to experiment with adding an ideal half brood box to run a brood and a half on a few hives to see if it’s more productive.
You can read about my mothers flow hive at Semaphore here: