I don’t have much experience with guineas, the only ones I’ve ever been around were a neighbor’s when I was a kid many decades ago. They totally free ranged, slept in trees, and never went into a building of any kind. I’m not sure how guineas will fit in this. But I’ll try your questions.
(1) I want to put in 5 roosting bars, how far apart should they be and how high.
The way I determine height of roosting bars is to decide what the floor level will be with bedding, then position the nests. Then I position the roosts noticeably higher than the nests. In a coop your size 12” should be plenty. I generally like them as low as reasonable but still higher than the nests because the higher the roosts the more open space they need to fly up and down so they don’t bang into walls, nests, feeders, or waterers. In a coop your size that’s probably not an issue. For chickens only a general rule of thumb is the 12” separation mentioned above. I’d expect that to work for guineas also.
For chickens only, many people build ladder roosts, with a roost near the ground and each other one gradually going up and out. A lot of people build them all the same height. Both can work. I’d expect the guineas to be dominant over the chickens, they will sleep wherever they want. The chickens will take second best. I keep getting hung up on the guineas since I don’t know how they will fit into the mix. Have you been on the guinea fowl section of this forum to see what they say about mixing guineas and chickens?
(2) where would be the best place to put my vents, up high or down low or both?
In Oklahoma you will have some pretty cold winters and some pretty hot summers, like you are seeing now. In winter you need vents up high so any breezes from your winter storms blows right over the heads of the fowl as they roost. You could write a book about all the whys of this but vents up high allow a good exchange of good air for bad without subjecting them to a direct wind. Your guineas and chickens could probably survive your winter weather sleeping in trees as long as they have the ability to move around and get out of direct winds, much like many of the wild birds do that overwinter. Of course you have predators to consider. They have great ventilation sleeping in trees. Ventilation in winter is important. This is another consideration I setting your roost height. The roosts need to be low enough that winter breezes from your vents pass over their heads.
The heat of you summer is much more dangerous than your cold in winter. Heat can kill chickens. In summer you need ventilation up high and ventilation down low. A wind hitting them is not a problem. Windows at roost level or below covered with wire for predator protection are great. I also have a large vent area just above the ground level on the cool side of my coop that I can block in winter if I want to. In your summers you can’t have too much ventilation.
(3) What is the purpose of hanging the food and water.
You don’t have to hang the food and water, but many people do. I hang my food but water is on platforms built up. As stated above, it’s to reduce the amount of bedding and other trash scratched into them. The general recommendation is to elevate them about the height of their backs.
(4). Should I build a ramp up to the roost bars.
If you have Silkies or other chickens that can’t fly, you probably need to build a ramp. If you have extra large chickens, chickens on the big side for their breed, it might not bad a bad idea. Extra large chickens, like those bred for show, can possibly hurt their legs jumping down from roosts. They may have trouble flying up too. But most chickens can easily fly to and from the roosts as long as they have room to spread their wings and fly. If you got yours from a hatchery this should not be a problem. My roosts are 5’ high and my full sized dual purpose chickens have no trouble getting up or down. From watching them they could easily go a lot further.
I will have 4 nest boxes 12x12x12 should I build smaller ones for the bantams.
You can build smaller nests if you want. The bantams might or might not use them. The larger chickens might or might not use them. I once used a cat litter bucket with dimensions 7-1/2” x 11-1/2” as a nest. Full sized chickens used it. It worked OK for egg laying but when a broody hen hatched chicks in it, it was too small. The first chicks to hatch climbed up on Mama’s back. When they fell off some missed the nest and went all the way to the floor. I retired that nest.
12” x 12” will work for the nests. It is generally recommended as a minimum size for chickens, I don’t know about guineas. I built mine 16” x 16” as it was easier because of my stud spacing. I like the extra size when three hens crowd into there to lay at the same time, even with other nests empty. And baby chicks don’t fall out when they hatch. Don’t sweat this kind of detail. Your nests should be fine.
There are very few hard and fast rules about any of this. We all do these things differently because so many different things work. Try to make it convenient to you. Your chickens and guineas will probably adjust. Good luck on your recovery.