Very nice tank! I think the duncans will be fine, just keep an eye on it. I have a dendro & 2 large hammer frags in my tank with seahorses (was an emergency with other tank). No its not recommended & I wouldnt suggest it to anyone. I may just be lucky, but havent had any problems with them. Since you already have them in there, just keep a close eye on it. If the horses look like they wanna try to hitch to them, or if they are small, then I would move it.