if you have a horde wow character can u make a alliance death knight in a different server?

The rules for which side you can make characters for do not change.

Currently, you must have a level 55+ character on the server already to make a DK. Originally, they were only going to require you to have a 55+ on any server to be able to make 1 DK on every server, but they decided to limit this so as to prevent a flood of DKs worst than what we liely already will get. They may eventually loosen this up later back to their original plan, but that is not certain.