Just checked the .pem I was given (by a person, not by Amazon) - looks like it was an RSA format key all along. I'm a little confused - it's possible I'm an idiot.
I'll check to make sure I'm an idiot by seeing what I can generate from Amazon itself.
They still don't seem to work with the app though. I've tried multiple approaches and it keeps claiming the key was rejected by the server. The ssh log only shows:
Apr 14 21:20:51 ip---- sshd[31560]: Received disconnect from ---------: 11: Normal Shutdown. Thank you!
I'll check to make sure I'm an idiot by seeing what I can generate from Amazon itself.