In general, there is no need to worry about bacteria from small animals or their food insects harming plants. They would never survive in the wild if there was. If you are going to add any kind of lizard or frog or whatever to a terrarium, please make sure you have good information about the specific diet and environmental needs of the animal and know you can provide them BEFORE you acquire one. I'm not an expert, but some lizards eat plants, and many would probably prefer a warmer drier environment than a terrarium suited for orchids - not good choices. I'm sure there are many good choices, just make sure you aren't going to kill the animal or the orchids.