Hey brhebert!

rats on your first Vivarium! It can be addictive. If you do feel an addiction setting in let me know. I have the perfect cure

Attaching plants to a cork background can be done in many different ways. You can pin them to the background using floral wire. Bend the wire into a U shape and press the ends into the cork clamping around the plant to hold it in place. You can us little dabs of hot glue, tooth picks, zipties, really anything as long as it gets the job done and is non toxic and non corrosive.

Collecting items from outside is perfectly fine, though you will want to prepare the items for use in your tank (don't want to bring in Natural parasites or bacteria that could harm or kill your frogs

For wood, leaves and rocks it is best to boil them in water for 30mins then place them in the over at 225 for a couple hours. This will ensure any insect, parasites, bugs, or other nasties meet the untimely demise they deserve

For plants you will want to rinse them well under tap water. Clean the roots free of any soil and then soak the plant for 5 mins with it's root submerged in water. then submerge the whole plant in a 10% bleach 90% water mixture for 5 - 7 mins (leaves, stems, and all). This will kill anything living in or on the leaves). Then rinse the plant and make sure the bleach water is all gone. Pot and let it grow outside the tank in ABG mix (a very common terrarium soil) for about a week or 2. Then Plant it in your tank and enjoy!

Moss is a bit trickier. Moss in the US is not typically tropical moss and does not always fair to well with Vivarium life. You can try using some, but most always it dies off. You can buy some terrarium moss from NeHerp, Josh's Frogs, or Blackjungle to name a few online retailers. I personally love NeHerps moss, but others have their preference as well.

Don't know anything about fire belly newts sorry :/

the 18x18x18 tank you have will do great for 2 - 3 frogs. The main thing you will fight with is tank height. If you have a water feature like you plan you will likely have 2" of water minimum, then 3" of soil. This cuts your tank height down to 13". So keep this in minds as you are planning the water depth and plants.

Let us know if we can be of any further help!