In most cases, a Mac mini will do fine as a server for both Macs and PCs, though it depends on how you intend to use it.
For instance, a Mac mini uses standard connections like IMAP for mail, SSH for remote login, etc. So, on the side of a client, it doesn’t matter what is on the other end.
A Mac mini will not make Mac software work on a PC. Though, you should be able to take over a Mac mini with VNC, and then you can control the Mac as if it was sitting in front of you.