Methylene blue would be the better option, especially if there are fry in the tank. It may damage some of your plants, so you may want to remove them during treatment. Do not mix the mentylene blue with the melafix, they are not safe to use together. The methylene blue shouldn't harm the shrimp, but if you're concerned, you may want to move them to a quarantine tank with your plants.
At the end of treatment be sure to add carbon to the filter to help remove the medication. Follow the dosing directions closely, be careful not to overdose, but under dosing means it will be less effective. Whatever measuring spoon you may use for dosing the medication will need to be cleaned afterwards with bleach water and reserved for medication use only, it will no longer be safe for human use.
The biggest risk is fungal infection for something like this, though bacteria can also be a culprit. Keeping tank conditions in A1 condition will help to avoid both. Provided it is just scales that are damaged, they can grow back. If the injuries go deeper, there is the chance she could bleed out without you ever noticing it. Unfortunately, there is no way to prevent that from happening if the damage is severe. I wish you the best of luck!
Let me know if there's anything more I can do to help.