it depends what kind of fashion runway you are referring to?
- the real proper fashion runway?
- or the normal kinds that are openly available in shopping centers?
if the latter, the lighting might not be ideal, and a flash might do you more good, but then again 500D+18-55, you might not have enough range.
Ask yourself which you need more
Fast lens - stop motion, do not affect ambient lighting, does not distract the subjects, thinner dof due to large aperture used
Flash - stop motion, affects ambient lighting (but can overpower ugly color cast caused by lousy location lighting), distracts subject, you can stop down your lens for more dof