i hear this all the time....i would put it in at least a 20 long....
no here is the thing....every puffer is different such as i have 2 puffer that i can keep in my community tank...a figure 8 and another puffer that is 4in....the tank mates are normal no aggressive tank mates such as betta, tetra, dainos, mollies and .....
now this can not be done with all puffers as i have seen some that can have no tank mates at all as they kill every thing in the tank...
so every one is different
now as for making it brackish not all figure 8's do best in brackish...in the wild the are found in high brakish to closed off freshwater lakes with no salt....such as my figure's does not tolerate brackish water well and gets stressed so i keep him in fresh...
EDIT:
i kept mine in freshwater first and then used a spare tank to see how he would do in brakish...and like most puffer they show stress by not being as active and turing gray.....again all puffers are different...if you want to wait it out in fresh as most live fine there whole lives in fresh thats fine...if he starts to act odd then you may need to convert to brackish....
