it is true a separation between the antennas of the two different bands (900 and 1800) is needed. It is about 1.5m for a ‘vertical’ separation as far as i remember.
You’d rather put the 900 antennas on top, because they’re meant for coverage. Therefore putting them on top give them even more coverage.
And finally, you can locate antennas from the same band just few centimeters apart. It is OK. If you have a single pole, you can place your three 900MHz antennas at the same level, each of them facing a different direction (0/120/240). And put the three 1800MHz antenna 1.5m lower (bottom/top separation) with the same configuration.
You will also need antenna supports (clamps and stuff) to perform this operation. Check which one will let you do such a mounting.