If you aren't planning on basing your entire tank around one, I wouldn't reccomend getting one. Especially not when he's under 1 inch to start.
While them releasing their toxins are not common in the hobby, it is possible. A Cowfish is reccommended to be the first fish in the tank, so it can establish it's territory, then tank mates should be peaceful fish that will not bully or chase him. From there, they grow extremely quickly. Expect 6+ inches of growth in the first year.
They cannot tolerate high flow, so if you have a demanding coral tank, your powerheads will blow him around to death. I run my MP40's under 50% now that mine is around 8", but previously I had to run 20-30% throughout. You also probably won't find many people that are willing to just 'buy' your Cowfish when he gets too big, for the aforementioned reasons. I have a zoo here locally that would be willing to take one, but my LFS had a hard time finding a home for theirs when they decided to get rid of him. Not a lot of people have 300+ gallon tanks and are willing to take in strays..
They are also very disease prone, can't be cleaned by cleaner shrimp, and can get stuck inside of rockwork if they squeeze inside due to their hard bodies, and horns.
They're considered an "expert level" fish, and if you aren't going to do it right, I'd probably say just don't do it.
