i wrote you a BIG answer, but it got erased.
the answer is in 3GPP, i’ll give you the link tomorrow (there is a special work task about UMTS 900, and the guard band you should keep with a GSM 900 system)
the minimum is 2.8 MHz separation (if surrounded by macro cell GSM900 using the “border frequencies”)
the minimum is 4.8 MHz (if micro cell GSM900 are using the “border frequencies”)
altogether, by using 4.8MHz separation between the two bands “borders” you’ll obtain the best results