Just remember that FBTs are slightly toxic. Anything that you place in there with them will absorb that toxin. It might take a while, but it will eventually shorten the life of whatever fish you put in there. Goldfish I would really not recommend as they create a lot of waste and would require you do more frequent water changes that could cause stress to your toads. I've seen shrimp harrass fish, and I can imagine they would do the same to anything in the water-they can also be sensitive to toxins or extreme changes in the water. Pleco I would aslo advice against. Plecos will easily outgrow the shallow bit of water of your half filled tank and like more room to swim around and explore. You can get certain kind of plecos that stay on the smaller end (ie. clown and rubber plecos) but again, it comes down to the toxins from the toads. If you want more action in the tank, add another toad or two.