My understanding of Tourny ruling is that they can't have any third-party parts on them (so sites like Anvil or Zealot are out for conversion pieces)
It REALLY depends on the tournament. If in a GW shop, they do have to be Citadel (or at least Forgeworld) models and bits. Those not directly in a GW shop may have different standards, but they usually will define how far conversions and "counts as" go if they are any size. If just local at a non-GW shop, talk to the TO.
In many cases, unless the TO has a vested interest in using your models to generate sales (i.e. GW shops), they are okay with such things, so long as they are consistent and recognizable. Using Cultists and Catachans as Kroot stand-ins would make sense, so long as you don't have CSM or AM Catachan detachments in the same army.