That’s a lot of questions. You sound like someone that likes to plan and get things right to start with. That is a great attitude. You might want to look through the Learning Center at the top of this page. There is a lot of help in there.
Some people like to give magic numbers for how much room you need for chickens. You’ll get anything from 2 to 20 square feet per chicken, some including the run, some not. The thing is, all these numbers are right for certain people. We are all unique in our set-ups, goals, flock make-ups, management techniques, climates, and all that where no one number will cover us all. I find that the tighter I pack them the more behavioral problems I have to deal with, the harder I have to work, and the less flexibility I have to solve issues that come up. To make my life easier I give them as much room as I reasonably can. You might want to follow the link in my signature to get some of my thoughts on what some of these variable factors are.
I think you will have plenty of room there for 25 chickens. If more than one of them is a rooster it could be tight. That’s another problem in talking about room. It’s not a case where a certain space is paradise but a tiny bit less is a total disaster. It’s a gradual change where as it gets tighter the tendency for problems goes up. Two roosters might get along great in that area or the proximity might lead to a lot of fighting. No one can tell you for sure until you try it with specific roosters.
I don’t know how you plan to lay out that area, coop and run. With 25 chickens I think you need a walk-in coop. You need to be able to access all areas in your coop. I just don’t see how you can do that for 25 chickens without being able to walk in. Make it big enough so you can clean, gather eggs, do maintenance and all that from inside instead of trying to squeeze into a tiny place.
Those RR ties are not a great fence to keep your chickens in. They can easily fly a lot higher than four feet if they want to. Since the top of those ties makes a great landing area, they will want to. Chickens live to fly up and perch on anything that looks like a place to perch on. Who knows which side they’d hop down on? They will not try to perch on a wire though, it’s too thin. What I suggest is put a short, maybe 2 feet high fence on the inside of the top of that wall to keep them from flying up there to perch.
A lot of people do use rainwater for their chickens but there may be a few issues with your plan. You need to keep the run dry. I don’t know how you are planning on setting up your gutters, downspout, and trough, but where does the overflow go? Away from or into the run?
The water in the waterer needs to stay clean. Chickens poop a lot and they will perch on the edge of the waterer and poop in the water if they can. That can get pretty filthy pretty fast. You need to be able to clean it out.
Another thing to consider is that mosquitoes breed in standing water. You need to either keep mosquitoes out of the water to stop them from laying eggs or dump the water regularly to kill the wigglers. A way to keep wigglers out of the water, especially rain barrel water, is to use dunks. These are an organic way to kill the wigglers without harming the chickens. I may be wrong but I envision you talking about a gutter and downspout that empties into a trough. Instead I’d suggest a rain barrel system where your use stored water to feed a waterer gradually as the water is used and you control mosquitoes by covering the rain barrel with screen so mosquitoes can’t get in or use dunks to kill the wigglers.
Do you need to lock the chickens up at night to protect them from predators? I strongly say yes. Most predators will hunt day or night. Some are more active at night but there is an active thread right now about bobcats often hunting during daylight hours. Foxes, coyotes, raccoons, and many other predators hunt during daylight hours. Dogs are probably the biggest threat for most of us and they certainly hunt day and night. But nighttime is normally your biggest risk. Some are more active at night but they normally have more time at night because there is not as much human activity to scare them off. Some people can and do go years without an attack even if they don’t lock the chickens up at night but most can’t. You can easily be wiped out your first night or first week if you don’t lock them up. No one wants to wake up in the morning to a bunch of dead chickens.
Different people use different beddings, wood shavings, wood chips, straw, hay, sand, or dirt, probably some I’m not thinking of. What works better will depend on how you manage it and how dry it stays. If the coop stays pretty dry you can use the absorbent bedding to absorb the moisture from the poop. If it gets wet you want something that will drain pretty well and the moisture will evaporate. Where you are you could build an open air coop which will let the rain blow in so you would want a floor that drains and dries. If you build a coop that stays dry you can use a whole lot of different beddings.
That’s part of the problem with chickens. There are so many different things that can work that you have too many options. If I tell you to do it a certain way someone else can tell you to do it a different way. They are as right as I am. Both ways can work plus things we did not mention. Keeping chickens isn’t that hard but it is easy to get confused. Good luck and welcome to the adventure.