Hi Alice- I would use standard gauge strings: gut in the 3rd, 4th, and 5th octaves, and nylon in octaves 1 and 2. You could if you want use gut in the second octave as well. It kind of depends on the particular instrument and what you prefer sound-wise.
I have started experimenting with using lever gauge gut strings instead of standard gauge, on some pedal harps that I have and I really like their sound. The lever gauge strings are about .002(read 2 thousands of an inch) thinner than standard gauge, so it takes slightly less tension to get them up to pitch. But what I like about them is that they seem to give you a cleaner, more clearly defined pitch when you release the string. You could try that if you want.