Welcome to Librarium Online!
I am proposing to do a rather extensive chariot conversion for my WHFB slaanesh chaos army. I'm going to use an archive chaos warrior with flail and hood for the sorcerer, he's going to be mounted on a khemri chariot, converted with chaos sigils, then i'm going to models chains onto a chaos spawn which pulls the chariot.
I was going to to use the spawn to account for the two steeds attacks, the warriors attacks and a spell familiar for the sorcerer, i was just wondering if this was legal? could someone tell me
Sure. As long as you use the same stats for the chariot you can make it look like anything you want. So your spawn will be a little weak, only as strong as two horses, but it's totally legal.
Oh, and to keep the real hardasses from whining you should also make sure it all fits on a chariot base. Other than that, go crazy.