Can you limit data for a number of apps and leave others unlimited? I've got a monthly plan that gives me 100mb of data for use with any app except for WhatsApp, Facebook (messenger) and Twitter, which are unlimited and will not consume my data. Is there a way to limit the cumulative data usage for all the other apps while leaving the other four without a limit? 
I've already restricted apps in the mobile data and installed no root firewall, but I'm looking for something more specific that will limit my data consumption when I've hit 100mb with everything else except the apps I mentioned before. 
I have an Xperia Z3 running Android 5.1.1 unrooted
There are multiple approaches you could take:


You could restrict background-data for selected apps. That way they can only use your data plan when you're running them in foreground. See: How does “Restrict background data” work?
You could use a Firewall app to block selected apps completely from accessing the internet. Usually those apps distinguish between mobile data and WiFi, so you will have no restrictions on WiFi if you don't want to. Though most of these solutions require a rooted device, there are also several solutions for non-rooted devices, e.g. NoRoot Firewall.


Setting up a "cumulative limit for selected apps" has no easy solution AFAIK. I won't say it's not possible – but still, I know of no such solution.