The lack of feathers can also be caused by having the wrong food. try adding some protein to the daily crumbles, I use hard boiled chicken egg that is crushed together with the shell. The hens need the extra calcium for the shell to have good eggs as well so kills two birds with one stone so to say.
Also I agree that is too many birds in that cage. these look like button quail, Chinese Blue Breasted Quail, and they are best kept to one pair per cage /enclosure although some people do keep them in groups with lots of hiding places and nest areas. The best way to tell the males from the females is to look at the vent area feathers, almost all males have red tinted feathers there and also a bib under the chin area. I do see a few silver birds and these are harder to sex. I would guess that the ones with all the feathers are males at this point and the ones missing feathers are female.
The yellow crust could be the start of infection so I would clean the area and use some antiseptic like Neosporin or the like but do not put too much as they will try to eat it and that's not good. It might just need to be cleaned off.
Do give them a pan of clean sand or dirt to bathe in and they will be fine.
good luck these look to be some nice birds and should do okay if you give them the right diet, clean water and care.
And I forgot to add that they mate at anywhere from 6-10 weeks old so not to young to be over mated now.
