The book says snowflakes need a min. tank of 30 gallons but they get almost 30" full grown so I personally tend to disagree. Plus they're messy eaters so keeping your water quality up with one in a 30 gall. seems like it'd be a real chore. Just myself, I don't think I'd go for one without at least 55 gallons or better unless you want him to be the ONLY critter in the tank.
They're generally harmless as long as you keep them well fed. Otherwise he'll try to nab your fish and go especially crazy when you feed your fish. I accidently had a hungry one in with some damsels at work once. I put the fish food in the water, he smelled it (their eyesight stinks but man, can they smell!), came out and went crazy trying to munch a fish!! I had to scramble to give him some krill before anyone got hurt! Whenever you go to feed the fish just make sure that first you hand him a few good chunks of meaty things (with tongs or a feeding stick!) to keep him busy and happy while you feed the fishies.