Love your names for the fish!

As you probably know, the Green Spotted Puffer Fish or Tetraodon nigroviridis is a brackish water fish from South Asia. It's temperature requirements (75-82F/24-28C ) are  a little higher than what is comfortable for Neons and Corys at 70-75 F/21 -24C. Also, Neon tetras and Corys come from quite soft water. I don't think they would thrive in salted, hard water.

Some puffers are pretty good community fish. Others are fin nippers. so watch him. They may also nibble plants.

Also their mouths (top and bottom tooth actually) continue to grow. Like rodents, they need something to chew o to wear down that biting edge. Feed them pond snails once in a while. If you had them in a genuine brackish water tank, I wonder if a small piece of coral would be useful for recreational activities.

See also sites like:
http://www.thepufferforum.com/forum/

There used to be a pufferfish home page. I can't find it now.

When you mention getting either guppies or goldfish, I'm pretty sure you mean either or. Their needs are so differrent it terms of water temperature, food and size of tank. 3-4 goldfish, which given proper care can live 25 years, eventually will be comfotable in a 55-gallon tank.