If you want to keep it in the fridge, cover with plastic film while still hot. This will trap a little moisture underneath and stop your rice pudding developing a skin as it cools.
Remarks
Combing two different types of chocolate has the advantage of adding a distinctive flavour (dark) and creamy sweetness (milk), but if your taste veers more towards one than the other, use whatever proportion you prefer.
If you like, you can also add raisins, it's just a matter of personal taste.