Will parts from a 2007 Ford Focus interchange with my 2003 Ford Focus SE?

You’re talking about two distinct models of the Ford Focus, so if there’s any interchangeability it won’t be much. Perhaps there would be some mechanical parts in terms of the engine, but the 2007 model was a physically larger can than the original (2003) so even things like the suspension units and brakes are unlikely to be interchangeable.

As for body panels and interior fittings, I doubt there’d be any interchangeability at all. The later car had different panels and the interiors were also completely different. Changes to styling also mean that headlights, tail-lights, glass and even detail things like door handles would not be interchangeable.