Just my observation, but my eastern newts seem at their healthiest, being most active and interactive when I can keep the water temp in the low 60s. That is usually in the Fall/Winter months. I personally try to maintain the water temp around 72F in the summer, with the occasional few day where it might go over 75F).
I don't know much about tree frogs, so I don't have much to comment about them, but I would think that tree frogs should be able to tolerate lower than ideal temperatures (for tree frogs) much better than newts would tolerate higher than ideal temperatures (for newts).
I've kept guppies with eastern newts before. I have read many comments about how much fish stress out newts and generally live in 'fish-less ponds'. I've found that newts will stalk, and endlessly follow guppies. My only issue was that if the newts don't eat the babies the guppies multiply too fast and start making twezer feeding a necessity for the newts to get food before the guppies swarm in. They were feeder guppies so my ACF tank go some live snacks once they became too much of a nuisance. As for fish-less pond, I honestly don't know if those exist in the US, and I have personally found newts consistently in ponds and lakes with large populations of aggressive predatory fish. One pond in particular has several species of predatory fish in decent numbers while still having a huge newt population.
Corey, you seem set on keeping the tree frogs with the newts, so I'm going to try to give you some constrictive advise under that premise (I think the other users have the species mixing covered as it pertains to the tree frogs and I agree, but they've already covered that). I would urge you to try to keep the water temperature at least around or under 75F which is the max recommended for eastern newts. I think mid to low 70s is considered the 'night time' temp for some tree frogs which isn't ideal for them as an all day temp, but I think it being a little colder for the frogs is better than consistently stressing out the newts with temps above their long-term tolerance. As for your question regarding plants, whatever grows and is healthy in your tank will be good for your newts. I would suggest getting a small portion of plants coined as being easy to grow and see how they do. Some plants that I think would be good starter plants are: hornwort, anacharis, frogbit, or water sprite. Once you have a little success getting any of those to grow and get established then you can try adding other plants that you like, but do your research about what those plants require. The ones I listed should do ok under a variety of conditions.