Admins can be members of the staff group. The only thing that differs from standard accounts is that admin accounts are members of the "admin" group. Being a member of both admin and staff should not affect a thing, and is in fact the default behavior. All accounts are members of "staff," with admins simply being additionally members of the "admin" group. What is the permissions setup of your FTP directory?
Being third-party server software, if your port and address setup is done correctly, then its going to be some configuration issue with the software. You can try contacting the PureFTPd developers to see what options they suggest, but this server daemon is separate from Apple's supplied options.
If you enable Remote Access then you will enable access via sftp, which is an encrypted and secure FTP service built into OS X and interfaces with the OS X accounts directory so you can give access to local user accounts instead of maintaining a separate authorization list. This is just another option that perhaps might be useful for what you are doing. Alternatively, you can enable the classic FTP server in OS X to do a similar thing, but this will not be as secure of an implementation.