River sand or in Urdu 'raith' is a good substrate for granular stuff.
I have seen a lot of beautiful bonsai from Kumquat online, however I have not grown one myself.
My suggestion is that, since it is another citrus tree it should be treated like one. Research on Google about what kind of soil citrus trees prefer, how much watering is good, how much light, and also see if you can find out how to prune it properly.
- Aman