Can I set all tasks in API to the highest priority level?

No, only two tasks can have PRIORITY_LEVEL0 (highest), all other priority levels can be shared. ** PRIORITY_LEVEL0 is a level above most system tasks and should be used with care. All other priorities (1 - 9) are lower than the system tasks and can run longer, with the condition the idle time remains above 0. How much processor resource is available when it is being shared with system?