First off, welcome to the forum
I started with red eyes too. They are so beautiful. They should be a breeze to take care of, just make sure you ask questions.
1. For new froglets, what do you keep them in that is simple and cheap for a grow-out? (critter keeper, sraight into a large terrarium)
How big are they? I think they would be fine in a 10 gallon for now. After that, I would try to go 10 gallons per frog. You can pick up a 10 gallon aquarium with a lid for about $10 or craigslist. For the humidity you can cover some of your screen lid with saran wrap.
2. is there a combination of sexes that is not OK ( 2 males, 2 females, ect.) and when does that start to be a problem?
I dont think there is a problem with keeping them together. Honestly, they sleep all day and arent active too much. I have never heard of red eyes fighting but if you do happen to see one being bullied then I would seperate it. I highly doubt you will have that problem though.
Also, if you are looking to breed them one day it would be best to have more males. So 2 males and 1 female would be great. It is hard to sex them when they are froglets still though.
3. I plan on making a 18x18x24 exo-terra for them when they are a bit larger, have you guys observed that a. callydrias has a preferance more for the glass sides (more of a euro- type terrarium) plants, or wood?
They pretty much hang on the glass for me. The other stuff is more for looks. They do like plant leaves also and places to hide. Pothos is great for red eyes. A 18x18x24 would be wonderful.
4. Is a water feature necessary/ beneficial for red- eyes?
A waterfall is not necessary but a water dish is. They will soak in their water dish at night or sit on the edge of it. Make sure you change the water every day
5. LASTLY- have you noticed color variation between individuals based on locale- not albino/ lutino vs. wild-type but belize/ colombia/ costa rica?
I have heard some people say they have noticed different colors. I have heard people say things like " those look like my costa ricans" etc Im not positive though.
If you ever have any more questions let me know
Id be more than happy to answer them.