1. Yes, sand dollers are perfectly fine, add all the beachy decorations you want
![Smile :)](./images/smilies/icon_smile.gif)
2. I believe if you check the care sheets, they have information on ecuadorians. What I can personally tell you is that they are aggressive, and need a LOT of substrate depth, 10 inches minimum, and more if you can. They also like saltwater more than PPs do, I believe, so you might see them in the SW more often than others might.
3. ABG mix is the standard soil mixture used for vivariums and terrariums, at least in the frogging world, and it's very nutrient rich. It's not necessary, but I threw some in the shower caddy just to give them some variety. I place leaf litter in there and they love to munch on it, just make sure it's sanitized and pesticide free. You could potentially plant stuff in the ABG mix, but you would need proper lighting (5000K to 6500K), and the crabs would likely eat any plants before they grew too much.
4. I used I believe 9 blocks of eco earth to reach 10 inches in half of my tank, but I also mixed in the eco earth with play sand in about a 1:5 mixture (1 part sand for every 5 parts eco earth). If I could go back, I would not have divided the substrate. In hindsight, I realize now that I am giving them half the area in which to tunnel and move when molting or digging. I worry constantly about it, and as soon as I have the majority of my crabs up, I'm going to try and rip the divider out. So you can create two sides, but don't put a physical divider between them.
5. Honestly, I just mix the poop in with the substrate. It would be almost impossible to get every little turd in there if I cleaned LOL, plus it's all organic. Also, I have a few springtails in the tank, which help clean up decaying matter like the poop, and I'm going to start adding more and more until I get a substantial population.
6. I couldn't tell you the best place to buy cholla, but if you do some searches, some craft stores sell nice, cheap pieces, and ebay often has good deals. Here's a website I have bookmarked: http://www.save-on-crafts.com/chollabranch.html
7. I had a pretty bad fruit fly problem recently, but I realized it was because I wasn't changing out the fruit early or often enough. I bought a small little trap and placed it on top of the tank. I vented the tank daily and changed fruit every other day, and they mostly went away.
Oh and I had an aqueon versa top for my old 40B, and it worked great, if it says 18x36, then it will fit.