As your tablet and phone will both be running Android versions newer than 4.1 then this is all built in, and the devices should automatically let each other know about this when they connect, which will let apps know that they should limit the traffic they use.
However you can also manually tell them that a particular wifi connection is a hotspot rather than broadband connection, just in case the auto-detection hasn't worked.
On the device that will be connecting to the wifi (your Nexus 7), if you go into Settings -> Data Usage and then the "..." overflow button you should see an option called "Mobile Hotspots".
Data Usage Menu (click image for larger variant)
In here should be a list of all the wifi networks that the device has connected to recently, with a tickbox alongside that you can select to say that the network is actually a mobile hotspot. This tells your phone to treat that wifi network as if it is a mobile data (eg 3G) network instead of a broadband connection.
Mobile hotspots (click image for larger variant)
(cribbed from my previous answer here)