I kept a 1 inch Long Horned Cowfish in a holding tank with 2 small seahorses for 2 months and they got along fine. I think the problem with them is they get so huge - Long Horn's get to 46cm, they are voracious eaters and judging from the little guy I kept behaviour I wouldn't be surprised if they became interested in the seahorses once large enough. I believe the poisonous factor is only exhibited in the presence of other cowfish, but that's just what I've read, can't say from experience. You won't find many occurrences of cowfish poisoning people's tanks in any case, just lots of resources that say that.
Here's some info I found:
"Surprisingly, seahorses are not a compatible tank mate for cowfish and boxfish. Seahorses have a highly specialized food requirement, and will have a difficult time getting their share of foods. Not only are cowfish quicker at nabbing food, they are more aggressive. They can even be intimidating, preventing the seahorses from seeking out food. Cowfish will also nip and pick at seahorses. Seahorses naturally grow a layer of algae upon their skin. Cowfish are attracted to the algae and the unusual locomotion of seahorses. They respond by nibbling on the seahorses, trying to determine if seahorses are a good food item."
Cowfish In Aquaria
I also found this article:
No Horsing Around Cowfish