Tootie, sorry, I haven't been on board for a while. I have experience with keeping these guys, including red claw crabs. I have a 10g with a red claw female and a 75 gallon palludarium with 7 fiddlers. If you wanna see pics, let me know.
On to your questions.
The set-up you have drawn looks good. In my experience, fiddlers don't spend as much time on land as one would think. "Fiddlers" can be a LOT of the Uca subspecies to their behavior might vary. Most fiddlers you get from LFS spend about 80% of their time in water. My land area is 24x9 inches and although there are "foot" prints all over, I have yet to see my fiddlers on it. They only come on at night. I say this because the most important thing is the WATER area, not the land area. So, if you had to choose, go at least 60% water and the rest land, because the more water, the more steady the parameters. It is hard to build a good palludarium [half land, half water] in a 10 gallon, I would recommend a 20 long.
Also, I would not get more than 2 fiddlers at the most for 10 gallons. Depending on their size, you could try your luck with 3. NO MORE than 1 male. So get 1 male, 1 female, or 1 male and 2 females. I had 10 fiddler crabs in my 75 gallon: 3 males, 7 females. I STILL had territoriality and aggression issues. The population kinda evened itself out and I am left with 7, which are much happier now.
A concern I have is the depth of that sand. It may develop anaerobic pockets which harbor toxic bacteria. No good. If I were you, I would probably build the incline with rocks or gravel, or just give the fiddler crabs access above water using tall rocks or decoration. I considered ways to do this for my 10 gallon and I have yet to come up with a good way to do it. I considered building some sort of floating or fixed device covered with sand, never came up with anything good. We have a lot of creativity here, I encourage input on this matter
Water questions.
First off, try to keep track of the volume of water you have, it will make things MUCH easier in the future.
What you need is brackish water with a salinity between 1.004 and 1.008. Unfortunately, hydrometers are inaccurate in this range. If you have the money, invest in a refractometer. They go for about 20-30 on eBay. I can recommend a good seller. But if you're careful with your mixtures a hydrometer will do just fine. To get the salinity you need, mix 2 leveled tablespoons of marine salt for each gallon of water. The salt I recommend is Instant Ocean, it dissolves very fast. Mix the salt in with your hand or something else, use your dechlorinator [I recommend Seachem PRIME because it is so much more than just a dechlorinator], let sit for about 10 minutes and check and see if anything has deposited to the bottom. If you see salt on the bottom, mix some more. In my experience, Instant Ocean is exactly that ... instant. I only have to mix with my hand for about 30 seconds, let sit for 5 minutes and it is good to go.
DO NOT vary your salinity. The crabs can take the swings, but your bacterial colony ... not so much. If you swing salinity you might see a mini-cycle and that's NOT good. So try to keep the salinity pretty steady. STAY ON TOP of your water level. Make sure that you compensate the water when it evaporates, with FRESH water, because salt doesn't evaporate. If you compensate with brackishwater, your salinity will rise. What I do is I mark the normal water level on the side of the aquarium and try to keep it at that.
Temperature
If your ambient room temperature doesn't go under 74*F, you are fine. Crabs like it between 74-ish and 84 IIRC.
Cycling
DO NOT get the crabs if your tank is not cycled. Invertebrates, including crabs, are VERY sensitive to ammonia. They will NOT survive the cycling process. Search the forum for information on how to cycle your aquarium if you need to.
Fiddlers are messy eaters. You will have to do partial water changes once a week, especially with a tank like yours. 30% should be good, you can go more. I feed mine once every other day.
BTW, what I have capitalized, I am not yelling. It is just my way of underlining important information
I am now subscribed to this topic, so if you have questions, I'll be around.