The way i normally run tasks on my main machine is:
1. During the night i close down most applications and set 100% CPU useage.
2. During the day i still allow the machine to use some CPU, about half of the cores.

However, multicore tasks mess up my work flow. What happens is that during the night BOINC downloads lots of 7 CPU tasks, and then during the day, even though i have set to use less than 7 cores those tasks, when started, do take 7 cores to themselves. I should really have control over how many cores should be used. Do i have to shut down network useage during the night, to disallow such a thing?

Just limit the amount of cpu cores that the multi-core cpu tasks use in the app_config.xml file.
Client Configuration
Scroll down to the bottom of the document to the Project-level Configuration and you will see an example for limiting the number of threads you want to use crunching MT tasks.
Just set the nthreads value to what you want. You can also set a max_concurrent for the MT tasks.

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