There are alot of factors here. Firstly a 1in cowfish is much more likely to be sucked into a wavemaker than an 8in cowfish. Additionally a 6,000 GPH wavemaker is much more likely to suck in a cowfish than a 600 GPH wave maker.
In general, cowfish are not considered (Reef Safe) so they are not often in aquariums with corals and wave makers. Having kept them in large fish systems with 6,000 GPH closed loops A small cowfish may be sucked into a large pump but a large cowfish can't get sucked into much. You will need to use some discretion to decide if your fish and powerhead will be compatible.
ive kept multiple small boxfish/cowfish (under 1.5 inches) with a nero 3 wavemaker on random flow mode (going up to 80% which is over 1000gph) and ive had no issues with them getting stuck, ive even seen one grab stuck food off of it.