I was not expecting this, but a reader here emailed in to GW customer service and got a reply on who can take Imperial Knights as allies. The answer was any 40k army......
I still expect something along the lines of a codex of sorts to come out this month, that clears this up as well. This definitely gives some good insight though into what may be the direction its going to go.
via a Reader here on Faeit 212
Here's a reply I got from GW Customer Service regarding the key question, can Chaos ally with Knights?
From: UK Customer Services
Date: 24 February 2014
Subject: Re: Imperial Knights
Thanks for the email, the rules printed in this week's issue of White Dwarf state that "They may also be taken as allies; you can include up to three Imperial Knights as a single allied detachment for each primary detachment in your army" As such any 40k army can take them as a allied detachment.
Perhaps the persuasive nature of Slaanesh has convinced a Freelance household to work for Chaos, or maybe they just pay better than the Forces of the Imperium.
We hope this helps and that you enjoy the new Imperial Knights.