I am considering starting a brackish aquarium. What plants and fish can I include?

My husband and I have several aquariums, but they are really just beginner tanks. I have a thriving semi-aggressive community in a 40 gallon, a betta in a 5 gallon, a tropical community in a 55, a molly/guppy nursery in a 10, and one single little green spotted puffer in a ten. He is small right now, only an inch, but I know he is going to get much larger. My husband and I purchased him quite by accident. He was labeled as a freshwater puffer, and the salesperson said he would be fine in our large community tank. After about fifteen minutes of watching him chase mollies and neons around, we decided to isolate him in a ten gallon, and do some research. As it turns out, we'd purchased a brackish fish that will require a really large tank as an adult. I do not relish the idea of buying a 30+ gallon tank for ONE fish. What fish species would be good tankmates, and what kinds of plants would you recommend? Also, what is a good resource for planning a tank like this? Thanks!