Mixing is very risky. I have a pixie and a pacman in the same tank, same size, but i will be separating them once they outgrow each other. I personally would suggest a vivarium because it keeps the humidity high and gives the frogs a place to hide. birds' nest fern and bromeliads are easily kept in these humid environments and are not toxic. make sure you start out with one frog because dart frogs are an investment and you dont want to lose hundreds of dollars because of a setup problem. Also make sure that the tank isnt humid enough to the point where the glass is fogged. get a hydrometer to make sure the percentage is constantly maintained. if the humidity is too high then his will cause bacteria and fungus to grow and will kill your friends in a short period of time. make sure the tank is humid and also aired out. I had to leanr the hard way about humidity problems. My first pacman frog was dead and had mold growing on it. I was at fault and vowed never to let another one perish because of my lack of knowledge and understanding.