the "cf." in Messor cf. barbarus stands for confere, the latin word for compare. So it maybe (or in this case very probably) isn´t this species but a comparable species that looks alike and nothing else.
I think that they will at least need some kind of short period with lower temperatures.
Messor normally grow quite quick and have quite big colonies and I know no exception of this rule. However a basin of that size will be enough for quite some time and when they can build their own nest and use the whole basin for that it will be enough for a long time. But there is no way to tell how long.