Flagtails usually do best when kept either solo or in a group of 6+. They are schooling fish in the wild, but they tend to pick on each other when kept in small groups. They grow extremely slowly, but they'll eventually hit 16"+ so it also takes a pretty large tank to house a school of them. I bought mine 6 years ago at 9", and he's currently around 14-15". They're a great solo fish though, and they can be kept with just about any other community fish, even small tetras. They're also a good algae remover.
The main tip I'd give is to make sure your lid is sturdy. They can be very skittish, especially when the lights come on in the morning, and they're very powerful jumpers.