you’re right, there is no short antenna with a narrow vertical aperture.
a microcell should not be dual band… it is a bit paradoxal, don’t you think ?
you must (re)define a strategy for the microcells in your network. it should cover an area small enough so that only 2 TRX can carry the traffic of this area.
Needing more than 2 TRX (for outdoor) means that your site is not really microcell… and you’re installing a macrocell, dual band : 1800 for capacity and 900 for coverage.
the area should already be covered with an umbrella. Why do you need so much capacity ? You should rather reduce the coverage of this cell, and add other microcells around.