When you place the machines, pay attention to the controls, handles etc. It is easy to measure the foot print and the make your set up on paper, and then find out in use you need to make adjustments, because the placement will have you smacking your hand when you operate a crank, or find it difficult to access a control panel.
In a small shop having to slide a machine over 6" may cascade as you begin to infringe on the next machine over. Also consider the moving bits, both for ensuring full range of motion, but also access to panels, outlets etc.
Milling machines in particular due to the largish table and table travel have a large footprint at table height, but they leave a lot of space below the table. I have some storage behind my mill, which at first glance seems awkward, but when I need to get to the storage rack, I can run the table to the right hand side which opens up a decent passage for me to get past to the rack.