mmm, ok, now this is clear 🙂 on my side, i don’t have any study… as UMTS 900 is still new, i’m afraid the few engineers that performed this kind of studies have low probability to come on this website and answer you.
if i were you, i would rely on drive-tests for the rural sites –> The distance between UMTS900 and GSM900 in rural areas (with “xx” km coverage per site) should be more than 2 times “xx” km. You might face some interference at the cell border, but closer to the antennas, the quality should be alright.
Beware that a GSM900 site can cover much more than 30km under the right conditions. The prediction tools are usually not reliable at such BTS-MS distances. Especially the formulas such as Okumura-Hata & co. That’s why a drive test is recommended.