Ssh – Same command on multiple servers

If you use the same public key on each system and the private key becomes compromised, then any system using that key, barring other restrictions, will be accessible.

I trust you are using password protected private keys?

In our management practice, we have low, medium and high security "roles". Each role uses a different key. High security private keys are never to be transmitted to external assets, used on laptops that could be lost/stolen, etc. Medium and low security keys can be deployed in a wider range of scenarios.

I suggest examining your usage patterns and see what makes since in terms of security roles. What is the damage done by getting your private key?

Have you considered placing your SSH private key onto a hardware device from which it cannot be stolen, removing the potential compromise of the key into a non-issue?

Both hardware security modules and smart cards can be used to store SSH private keys in a secure manner, enabling all cryptographic operations to be performed on the device, rather than on your operating systems. However, they are not a panacea, as these require backup hardware devices also, in case of a hardware failure.

Your public key has to be in ~/.ssh/authorized_keys on the target box. If you want more than one key in this, just copy them over to the target box and append them like this:

cat id_rsa.pub >> ~/.ssh/authorized_keys

Also, read this info about putty key compability and how to convert keys.