Hope you don't mind my commenting within your piece - it's because I'm lazy
The floor of the house is made of ply, (i think), he has asked if any of you thought painting it with a bitumen based paint would help to waterproof it.
NO it will be horrible to handle. You could use a varnish, but it probably isn't needed; a coat of diatomaceous earth slurry/paint is best IMHO. It acts to deter mites and poo is easily scraped off. It's cheap and easy to re-apply. No bad side effects. Try and make the floor itself removeable then you can take it out for a hose down and repaint in fine weather. We poo pick daily, so it never gets really horrible, but an air and wash are (not) good for bugs and mites.
He is going to make an elevated floor or perch type design for them to actually hop along so i dont think they will actually be walking on the floor itself.
WE have a floor under the perches - ply, treated as above and sloped at the same angle as the perches rise. This makes it easy to clean and, if any birds fall (which they do when trying to get all 14 onto one perch on mad nights), then they cannot fall far and can easily start again from the bottom.
Outside this floored perch area is a plank which runs the length of the coop from pop hole to nest box at the opposite end. They can get to the nest box without going onto the fouled floor. This seems to work well and the nest box area is kept clean - they're very fussy are our monsters!
The perching will be removable for me to clean the floor. There will be some other perches higher than the nest boxes for them to sleep on if they wish.
IF the nest boxes are separated by a part division, you can just put trays or big bowls in on the floor as nesting boxes. They need to be the right shape to prevent falling over with heavy birds, but you can put a weight in. Not too high washing up bowls are fine so they don't need a step. Shavings stay in for a few days and you can top up as needed. Again cleaning is easy. Shavings are not needed elsewhere.
They really should sleep on the perches, but if you have birds that can't as we have - an ancient old lady,- then an ante-room between the perches and the nest box area works fine. This also worked when we had two small newcomers and might be the idea for your small chicks. As they grow they automatically look for perches. You might have to put a divider in for a night or two until they get they idea that they can't sleep in the nest boxes.
Would it be easier to have a lip at the front of the nest boxes to stop the straw or whatever we use from coming out, or just keep it flush to the main part of the house, he is trying to keep everything as smooth as possible to give red mite less places to hide.
THEY'LL throw things out however high you make it
. Try standing at the side of them dustbathing - then wash your hair!
Would yard broom handles be suitable for perches.
AS said no; the CLS timber from B&Q which is 2x3 inches planed and with rounded edges is just right and cheap.
HTH