Hello there and
to the forums!
You've been given great advice above. You should always quarantine a new bird for at least a month before introducing them, both to prevent the spread of illness and disease from the new bird to your current bird, as well as to let the new little one settle into his or her environment with minimal stress.
The best idea is to take both your girls into an avian vet for a checkup
The vet will be able to give you advice about the toenail as well as make sure neither of them are hiding any illnesses.
Meanwhile, you've come to a great place to learn even more about the best for budgie care! Be sure to read through everything to ensure you're up to date on everything. For example, you should remove the dowel perches and replace all of them with natural wood perches in order to discourage bumblefoot and keep their feet healthy.
If you have any questions after reading through everything, be sure to ask as we'd love to help!
Hope to see you around the forums