The file format is the same so 13 server can host 14 files. There may be a few "new in 14 features" that won't work when hosted from server 13, you'd need to research or run your own tests. (When hosting 12 files from 13, most new in 13 features worked, but some such as "perform script on server" for obvious reasons, could not be used until the server was updated.)
I set a file to open only with FM 14 then I tried opening it on the FM 13 server and it didn't like it. So even though it can host the file and you can work on it in FM 14, you can't restrict it to 14 client i the server is 13.