Several hundred gallons? Seriously? Mixing can be done safely in a 29G tank. I'm mixing native species in a TWENTY gallon tank. I've got a tank currently that's around 80-90G, and I'm mixing in that. I've mixed in a tank as small as a 10G! In fact, the guys in the 80-90G tank just came out of a 50G tank (the largest tank I have, actually, that doesn't have a topper to maximize size). All of my mixed critters are certainly healthy and happy.
That said, it depends on the required parameters for the animals you want to mix, the risk of toxicity, etc.
Is your FBT WC or CB? What size will your darts be when you purchase them, and how large will they get? Nocturnal frogs are opportunists, they WILL and DO eat in the day time. FBTs will be up whenever they feel like it, and they'll likely occupy the same niche as the darts. I personally would not risk mixing with a FBT and darts. I've not kept a grey tree frog, so I don't know about their habits.
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