I have just received a Roridula from Sean Spence. He would be best placed to help you here. The plant I got from him was in a sand/gravel based mix (it is very heavy). He has recommended to me to keep watering to a minimum. We are just going into winter and I have been told to top water once a month, when the mix begind to dry out. In the warmer months they should also be watered only as the mix dries.
They need as much light as you can give them and also require good ventilation.
I am sure there are others with more experience with these than me that could provide better information but I think this will point you in the right direction. Cheers.