Coffe cups probably will be fine too. You never know which is more safe coffe cups or terracotta clay as to matereal, because nowdays who know what they use during manufacturing...(everything made in China). I agree, coffe cups look nice, I had one small cup with a plant in tank before.
They say terracotta clay lows PH, I will see if it is true as PH of my water is very high...
As to rough surface, mine are pretty polished. I also tryed to chose the best polished when was grabbing them on the store shelf. There is really no place that could harm on my pots. If you ever going to buy pots with hole, they say you need to close it with smth as Bettas can harm themself in those. As I said my pots at 3" have no holes and as to 2" ones the holes exreemly small - around 3-4mm.