Yes, the warmaster greater daemon works and i think that the minis from the slaaneshi character pack work as daemon princes (although i don't have them myself). There were also small greater daemons produced for epic at one time, which work as daemon princes in comparison to the larger greater daemons.
Smaller greater daemons are shown here, scroll down: http://www.galactic-intrigue.com/epicol ... rames.html
For seekers you can use the old chaos slaanesh beat riders or the currently available warmaster daemon steeds of slaanesh.
Knights – necron warrior legs look a bit out of scale (too large iirc) but it's what ill be using for mine anyway.
Debaser – well once i get back to my EC, my debasers will be walking gramophones but then it is a themed army
