The chicks you have are going to be more active than the day olds. With that said, the new ones could tire themselves trying to keep up to the older ones. You can put them together, but just be sure the others arent aggressive to the new ones.
Coincidentally the day my chicks were hatching my dog brought 2 ducklings and 2 ducklings that were still hatching(!!) they shared the incubator and spent their early life together and i never had any of them get sick. If they hatch at the same time there shouldnt be any behavioral issues...
Ive had bantams before, and theyve looked similar to your chicks. As for sexing ive never been able to tell when their young. Ive chicks with large combs Turn out to be hens even though large combs is a rooster thing. If you have any other questions Dont be afraid to ask, i might be able to help!
Clipping their wings takes away their only source of defense. If a predator gets in their pen theyll try to fly, but they cant if their wings are clipped, they cant protect themselves.
Eggs in a nest naturally lie on their sides, not their ends. Im with dotknott that ive only ever hatched eggs on their ends. But that doesnt mean its necessarly bad to have them stand in ends.
Fox attack at night is bad, but loosing a duck and chicken within a week from eachother at NOON from a fox hurts worse because your awake and werent there to protect the flock
A mean rooster is like a mean dog, it cant be trusted around children. I speak from personal experience when i say being chased by a rooster is extremely frightening.
Watch the other chicks that they dont peck her rear and cause her further discomfort. Sounds more an internal problem, ive never delt with that so i dont have advice as far as treatment
Have you tried adding oyster shells their diet? They could be lacking calcium and the shells might help with producing an egg. Ive found oyster shells at tsc, home of economy, etc.