Can you limit data for a number of apps and leave others unlimited?
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.