I recommend including the cafe space in the water use calculation so the total water use is more accurate - otherwise your water savings will be inflated. Since you do not know the actual fixtures that will be installed choose the same basic fixture for both the base case and design case so you are not claiming any water savings on those fixtures. If you do not know the actual occupancy of the space yet, estimate the FTE based on the maximum occupant load of the space (based on Fire Code for example).
Yes, as Rick mentions, you could also include requirements (or suggestions) for water efficient fixtures in tenant guidelines.