By allowing BOINC to run for several days with the zero and zero days setting. BOINC 7 has a new work fetch module and new separated CPU and GPU schedulers. It will have to learn anew about how you run things. It can't do that without starting with one task per hardware core per project.

It won't do it immediately, there is no way to force it. But your BOINC 6.10 or 6.12 didn't do this immediately either, it had to learn this as well.

So if you're just willing to let go of things, not look in BOINC Manager every moment of the day what it is doing, you will eventually see that it will do things the old way. Just one task per project per core, depending on resource share and REC (the new debt).

Back in the early 6.x release the Min and Additional work buffer fields were set to ZERO so that projects would only get a single task - not multiple. Then you would have many tasks but only from individual projects. How can we do this in the new 7.0.25 version?

I'd almost say guess... ;-)
But uhm, how about setting both to ZERO again?

The minimum work buffer setting sets the minimum amount of work you're going to request.
The maximum additional work buffer sets the additional days worth of work you want to have.

So if you only want 1 task for each CPU + GPU core, you set 0 + 0, which will fetch 1 second per hardware core.