It does look like Zwartkop to me, but give it a few months and see how it develops under your care. The color is pretty unique and distinctive. Any form of arboreum has the potential to be a large plant (say 10 inch or 3 gallon pot size by the time it flowers) but for now, and given the season, I wouldn't give them more than about a 2 inch increase in pot size.
Those are lovely plants, by the way.
