1 - Blossom should be alright. It's just the girls sorting out their ranks in the tank. One nipped fin isn't bad but keep an eye on her. If she seems to keep getting ganged up on or especially if she starts getting hurt worse she might be better off on her own.
2 - Again, one nip isn't bad and she'll be fine.
3 - In a 10 gal tank with artificial plants I think you're good at 6, and personally wouldn't add anything more. If you want to add more you should honestly look into setting them up with live plants to help improve the water quality and reduce their stress
4 - Corys may "clean" the tank but they also add to the ammonia build up. They won't reduce the amount of water changes. If you really want something to clean the tank you might want to look into a snail if your girls are already really aggressive but again, think about the water quality and how often you want to be doing water changes
These are just my personal opinions. I'm setting up a sorority in a 10 gall as well but I have been working on heavily planting it and don't plan to do more then what you already have