I'm wondering if I can make the assumption that I can get a file using scp on any server that I could get the file using sftp.
Mostly, yes. Both sftp and scp are file transfer protocols over ssh, and sftp is a more recent feature than scp, so while there are a few older setups with scp but no sftp, the converse would take some deliberate configuration.
However, it is possible for a server to allow sftp but not scp. One reason I can think of to configure a server this way is if you want to allow a single service over ssh, for simplicity's sake: sftp and scp provide the same level of access security-wise, but sftp is more convenient, so sftp is the preferable choice.
In other words, if the server can sftp, it can scp; but it might allow sftp and forbid scp.
No, you can't assume that.
scp depends on
SSH being available, which is not linked to
SFTP being available.
~~edit: more details~~
ssh lets you do more than just copy files. It will allow you to run commands. So it is very easy to imagine a server that would allow you to sftp, but would not allow you to ssh.