I'd like to make my own thai red curry and green curry pastes but both of these call for shrimp paste which isn't widely available in my country. I do have a packet of small dried shrimp (with heads/legs on). Can I use these to subsitute for the shrimp paste, or should I use fish sauce instead? or perhaps both?
Your dried shrimp are fresh, then dried. The taste belachan imparts has more to do with the fact that it is fermented, than is shrimp. Fish sauce is fermented, so you really would be better off adding both :)
Rehydrating the shrimp in some fish sauce with water, then adding it to your mix for blending would work best. You may not be able to blend your paste properly if the shrimp is still dried.