Most colocation companies charge per rack unit, 1 of which is 1.75" high. Some will allow you to have a shelf, and if you've purchased enough rack units, allow you to put into the cabinet whatever you please, so long as it's safe and remains within your allocated power consumption. If the box is too heavy for the shelf (unless the shelf bolts at both the front and the back), they will likely refuse to install it.
The fan situation may play against you though. Rack servers are designed to pull cold air in through the front, and vent it out the back. If the fans on this unit are oriented the same way (air pulled in through one end, out through the other), then you will need to ensure they orient the unit correctly in the cabinet to ensure the side that sucks the air in faces the cold aisle.
Hope this helps.