I also have my bettas in on gallon tanks. I have one in a vase that actually hold more than one gallon. in the tanks are just plain old gravel and fake plants, but the one in the vase has a bamboo shoot that goes up from the middle of the vase. He seems happy, healthy, active. Plenty room to swim, still do the water change, but I tell you the bamboo shoot ain't ever looked better. To answer one of the original questions, ghost shrimp will be fine with your betta, I had two in a two gallon tank. The only problem I ever had was when I first put them in the betta nipped at them once or twice, didn't hurt them. I think he was just checking to see if they were food.
:devil: