choosing priority of processes in linux 2.6

Hello,

I am working on an embedded Linux system (the latest kernel version
2.6.23) with 3 RT low-latency processes, and 5 processes with normal
priority. We do not use interrupts, but periodically poll for data in a
RT low-latency process to check when the data was written. These
low-latency processes should not run for longer then 5 ms, and the CPU
load should be about 50%. Low latency processes with run with
SCHED_FIFO, and normal processes with SCHED_OTHER setup.

Can someone recommend me where to read about choosing the priority of a
process?
I have read about process priority in man pages for sched_setsheduler
(3), but it does not say anything whether I can somehow make the kernel
non-responsive with choosing too high priorities.
Can anything go wrong if the chosen priorities are too high?

Thanks in advance.