What are the valid users that need to exist on Mac OS X For example with CleanMyMac username option I can see users like "Kerberos FAST Account", com.apple.calendarserver and large number of others all with system privileges  with id 260 or below except for Message Bus which is 500 MacPorts also has a user with id 503. Anything below 500 has a "system" in name. All "real" users that I have created are above 500. 

so what would be the list for valid users that I should see in the space below 500 in a standard Yosemite installation with OS X Server on it...
There are 87 valid users. 83 from OS X 10.10.1 (Build 14B25) and 4 from Server 4.0.3 (Build 14S350).

You can see them from terminal with the command:

dscl . -list /Users


The list is as follows:

_amavisd
_appleevents
_appowner
_appserver
_ard
_assetcache
_astris
_atsserver
_avbdeviced
_calendar
_ces
_clamav
_coreaudiod
_coremediaiod
_cvmsroot
_cvs
_cyrus
_devdocs
_devicemgr
_displaypolicyd
_distnote
_dovecot
_dovenull
_dpaudio
_eppc
_ftp
_geod
_iconservices
_installassistant
_installer
_jabber
_kadmin_admin
_kadmin_changepw
_krb_anonymous
_krb_changepw
_krb_kadmin
_krb_kerberos
_krb_krbtgt
_krbfast
_krbtgt
_launchservicesd
_lda
_locationd
_lp
_mailman
_mcxalr
_mdnsresponder
_mysql
_netbios
_netstatistics
_networkd
_nsurlsessiond
_nsurlstoraged
_postfix
_postgres
_qtss
_sandbox
_screensaver
_scsd
_securityagent
_serialnumberd
_softwareupdate
_spotlight
_sshd
_svn
_taskgated
_teamsserver
_timezone
_tokend
_trustevaluationagent
_unknown
_update_sharing
_usbmuxd
_uucp
_warmd
_webauthserver
_windowserver
_www
_xcsbuildagent
_xcsbuildd
_xcscouch
_xcscredserver
_xcsd
com.apple.calendarserver
daemon
nobody
root


For completeness, my OS X (non-server app) Mac has 84 users.

It has an additional Guest user (FileVault 2 added that one) and is lacking these server apps.

_xcsbuildd
_xcscouch
_xcsd
com.apple.calendarserver