I have been thinking about this thread for a while now.
Regarding the nozzles, liquid velocity limitation standards as pertain to any piping/nozzles, PHE or otherwise would prevail. You must have parameters for your piping and other components in the piping trains.
Regarding the plate packs, PHE's love velocity (within parameters, of course) but higher velocity means higher pressure drop, so more often than not the system and it's pressure drop limitations are the velocity limiters in a PHE.
If you can stand the pressure drop, go for the maximum velocity your PHE supplier will allow.
If your nozzle sizing is consistent with your proper piping standards (in other words, your nozzles are not inordinately oversized,) I believe their limitations will dictate the velocity limits long before you have to worry about the plates. Just my opinion. I did not do any research that you could have done, like Google, etc.
rmw