For a puffer such as a fahaka, i would probably go for a baby one to start with since they will pretty much grow to 12"+ in the first year, and aren;t as fragile as some might thing, given you give them clean water and a good diet...
The only thing about getting one that small is that you can't be sure it will grow up to be the potential monster you were hoping for, as it seems that even here on MFK, there have been a few members who reported buying a baby Fahaka, but after "x" amount of time its still only at around 4" - 6" which is unusual for a fahaka...