Side loaders are a great way to reduce your aisle width. The limitations that you need to concern yourself are (I am assuming the side loader your are talking about has a mast that will only extend side ways typically on the right side of trucks):
1. The amount of space required for a turn aisle possibly at each end on the rack system. This extra space takes away from your aisle space savings.
2. If the rack dead ends against a wall then the side loader must back down the aisle to service the other side..
3. If only one side loader is available for use - what do you do when the machine is down for repairs - sometimes it can be down for a long period waiting for repair parts (even during the warranty period). Does the dealer have a rental unit to keep you up and running? This is an issue with any VNA equipment (Very Narrow Aisle), tri-loaders, Bendi's, Flexis (articulated forklifts), turret types, etc. You need a back up plan.

Forkingabout comments are also very valid.