Need opinion on LFS professional

Hear me out: I currently have a 12g FW & I am planning on switching over to brackish.

I visited the fish store a couple of weeks ago and fell in love with some puffers there. They were about 2" long, really cute. They were mostly silver, but had some bright green on top and covered with little black spots.

I told the guy who owns the place that I really like them and asked if it was possible to house a couple in a 12g. He said I could keep 2. I also asked what kind of puffers they were and how big they would get. He said he wasn't sure what the scientific name was, but he called them "spotted puffers" and he's never seen them bigger than 3 or 4". I asked him what they eat and he said they'll eat just about anything.

So I don't actually know what type of puffers they were, but apparently this guy has been keeping all sorts of fish (FW, BW, & SW) for nearly 30 years without ill-effects.

I just don't know if my set-up is really good enough for them and I'm reading so many different things on the internet. I just don't want to make any mistakes or get into something that will be too difficult for me. I also don't know how much to trust this guy even though he's really humble and down to earth.

Any help or opinions would be greatly appreciated.

So basically it is out of the question to house 2 of those puffers in a 12g even while they are small?

Just out of curiosity, why do they need so much room with the 1 inch of fish per gallon rule?

Thanks!

Thank you, that helps! I opened the article, but haven't read it yet (at work, no time just yet!)