It wont require marine as it matures... It will thrive well in freshwater when young and then brackish is required when it is older. It isnt a necessity for it to be it marine... Its more of a choice. Mine did very well with lots of different tank mates from small to large and never harmed a thing! maybe i was lucky!
If I'm not mistaken and you're definitely talking of green spot puffers they get close to 7" and definitely require brackish then marine as they grow. The op decided on rams though so it's a moot point lol.