When it comes to night elf guilds, I can reccomend Dirge of Teldrassil. They take in death knights too.
When it comes to allied races, which ones are you most interested in? I´d say most of the RP guilds in WoW are centered around races, so you may be unable to join the guild you picked on your main with your alt. So, if the DK is just a stepping stone for another character, I´d suggest using her purely for the content and get the race you want to RP as quickly as possible (fortunately these days you should be able to get them quite easily).