In the tank where I had clowns they never once bothered the other fish at all (rasboras, neons, cats, dwarf turquoise rainbows, SAEs, ottos) Just played around. I primarily kept them because I like their personalities and antics but I did have a snail infestation that was erradicated.
I had crypts, java, anubias, lobelia, ludwigia repens, green rotala and hygro. Occaisionally, like maybe once every couple months I'd noticed several bites in the hygros as they have soft leaves. But that's it...and hygros grew so fast it didn't matter. Another thing is that I occaisionally threw in some algae wafers for the algae crew, and then the clowns might of filled up on them a bit--maybe the clowns weren't getting the veggies they needed in their diet??
I'm growing HC and glosso emersed right now and I'm not planning on putting in clowns because I suspect they do too much rooting around and might nibble at some of HC. Though I'm not sure as now the HC is firmly rooted into the substrate....
However I love clowns so much I think I'm going to setup a low light tank with java and anubia varieties and throw in JUST clowns. They grow quickly and get huge if treated right and they are fun to watch. Also always get them in groups of at least 3 or 4, they hide out and get reclusive and skittish with just a couple or one.