- Is there any processing time enough for that interupts ?
6000 interrupts a second is one every 166 uS. Since an interrupt takes around 5 uS to process that should be OK.
- Will the interupts process hit/break/intervere each other ?
That is a possibility. One interrupt can't interrupt another one, however the second one would be remembered, up to a point. Why not use the external interrupts, rather than the pin change interrupts?
What is the objective here? To work out the engine speed?
You can use timers to count events like that, which will be very efficient and accurate. I have some examples here: