Run few processes
Is it ok to run few hashcat.exe at the same time?

What if:

1. hashcat.exe cracks using GPU and other hashcat.exe cracks using GPU - will they collide?
2. one cracks, second is running to generate passwords by rules - will they collide?
Multiple sessions are fine. I regularly pause one long-running session to do a quick shorter session. You will want to use the --session parameter to give additional ones different names.

Multiple sessions actively running at the same time will compete for resources, unless you target specific devices (CPU for one and GPU for the other, or distribute to specific GPUs, etc.)