I use veg oil or sunflower oil, but I guess it's down to taste. Prawns, being delicate, I expect need to be added at the end of cooking - but I never use them so perhaps someone else can confirm. 6x whole chillis is enough to make a curry for 4x people fairly hot. If the curry was not hot then imo it's down to the chillis. I just use regular green chillis from Rye Lane market / Khans and they're sufficiently hot that 3x chillis plus a 1/2 teaspoon chilli powder is plenty for a curry for 6 people. This works for us and we like fairly spicy (without getting into Vindaloo territory). The recipe I posted the YT link to, says 6x chillis (I used 3x) and two TABLESPOONS of chilli powder (I used 2/3 TEAspoon) and it was well hot, it also says 500g chicken but I used 1500g chicken and it was still hot. Just in case you try that recipe - which I recommend as it came out excellent.