Hi Amber. Let me see if I can help you with a few of your questions.

First of all, congrats on your opportunity to get some frogs!

Now, for your tank size. With a trio I don't think I would go for anything under a 20 gallon. Once your frogs reach maturity there is a chance that there could be some aggression issues. It doesn't always happen.. but when you end up with a female heavy tinc trio it is common. Azureus are notorious for female/female aggression. Some people never have a problem.. but you would just need to watch them when they get older to check for any issues such as wrestling, thinning frogs, etc. Some of that can be helped by having a larger tank, properly planted, with lots of hiding spots and visual barriers.

As far as coco fiber, I woud personally recommend getting yourself some ABG mix. Not only would your plants love it.. but so will the microfauna (isopods/springtails). There are many places with it available online, or there are recipes to make your own.

Fake plants.. I would just skip that. Real plants are healty for the frogs. They help to keep humidity levels up from what I understand. I would just save up for the real deal. You don't have to go for anything expensive or exotic. The frogs don't care Be careful though. Some of the cheaper plants can take over your viv.. like pothos and wandering jew. That can be good and bad. It takes work to trim them back.. but you get more opportunities to look at your frogs and make sure they are ok while trimming.

I think that a 5 gallon tank or tub would be ok. If you had the option I think I would go with a 10 gallon though. Not knowing how long your viv setup may take.. you would want your little guys/gals to be happy and stress free which would give you time to build it well. As Lynn said, the quarantine tank can be as simple as you like. Damp paper towels are great so you can monitor their feces to make sure that it isn't watery and you can monitor their feeding as well to make sure they are all getting what they need. A simple pothos cutting is all that would be needed as far as plants go.. and a water dish for rehydrating.

Tincs aren't known for their climbing.. but that doesn't mean that they don't climb. I would agree that at 18" high they would have enough room to be comfortable.

Hopefully that helps a bit.