yeah, the upside to hydno's is that the M-filaments only reach a little way. basically, when something actually touches the hydno: LOOK OUT! but if you are daring and your frags are secured, they can be positioned close to other sps.
overall, they are righteous coral to keep. I also wish there was something called "MPS" medium polyp stony. this is where I would file: Hydnophora, Galaxia, Turbinaria, etc.