How can I force input to be interpreted as millimetres instead of metres? In Sketchup for example you can chose default unit. If you chose cm, you type in 17 and the line will be 17cm. Or you can choose to type in 1.7 and the line will be 1.7cm. Or you can change to metres and type in 3 and the line will be 3 metres. Or you can type in .3 and the line will be .3m.

When you're trying to type in precise measurements in Blender, such as 16.4mm X 12.5mm X 8mm, I have to either type the size in metres, or specify a different unit at the end of the number.

How do I tell Blender to assume that "7" means "7 millimetres," without having to put "0.00" in front of it?

Or is the only way to make everything in metres and treat them like millimetres  and then downscale everything by 1000 to make the measurements true to the plan?
You can do that from the Scene panel, Under Units, Scale 1.0000000 means that it's in meters, change it to 0.01, this will make your units in cm, or 0.001 will make them in mm.
Be careful though if you're planning to export your objects to a different software, like UE4 for instance, you need to change this later to meet the requirements of that software.


Edit:Make sure that you also change the scale in 3D View window, under Display (in the properties shelve to your right hand by default, if you can't see that, press N while hovering over 3D View window).
Check this answer too: Change the scale of the viewport