I got SSH working with SourceTree to connect to a private github repo using an alternate keypair (i.e. another one than my id_rsa).
This involved adding the following text to ~/.ssh/config:
And of course generating the USERNAME keypair I referred to this guide, which led to these commands:
ssh-keygen -t rsa -C "firstname.lastname@example.org" -f output_keyfile
SourceTree kept asking me for my password even after I set things up to where command line git was working right in the terminal. To fix this I had to format my repo URL with ssh:// in the beginning as suggested by Atlassian.