You can have them anywhere you want, but their permission and the permission of the parent folders need to be strict:
no writable access for the parent folder (for others and all)
644 for a public key
600 for a private key.
You then:
declare those different keys in a ~/.ssh/config file (example here)
change the remote url in order to use the appropriate entry of the ~/.ssh/config file which described the right ssh key to use.
change the remote url in order to use the appropriate entry of the ~/.ssh/config file which described the right ssh key to use.