I would definitely check out the threads in the vivarium portion of this forum. I got A LOT of ideas about building mine there.

Also, do NOT mix species. Even if they're the same size, they both emit their own kinds of toxins which, over time, will kill each other. If you want red eyed tree frogs, stick with them alone. I know that you can read some care sheets that say they get along well with other tree frogs, but the fact is that they get easily stressed and the toxins can and will hurt each other.

I would also suggest doing a vertical conversion to give them more height, there are places that sell kits. AAA Frogs is one of them. And you are right, do not use heat rocks, that can burn them. Placing a heat pad under the tank will help keep the tank warm and the humidity up.

Plants... I would suggest doing a google of tropical terrarium plants, that's how I got most of my ideas. Also, looking through the vivarium build threads will help. Make sure you use a layer of hydro balls (can be found at most pet stores or online) as that will help with drainage to make sure the plants stay healthy.

For lighting, I have a 50 watt blue daylight bulb on each of my tanks. I have gray tree frogs though, I'm not sure if a RETF would need something different.

Welcome to the forum and good luck!! You will find a lot of great advice here