I think if you need to ask a question like this, then you should seriously question if you have the qualities required of a professional pilot or flight instructor.
And coming from an instructor(!) that comment beggars belief.... the only stupid question is the question you
don't ask!
For an initial but
informal (read the post those guys advocating suits etc!) meeting (I always think 'interview' is OTT for some FI 'jobs') I would opt for the standard instructor garb of today which appears to be shoes, black trousers, white (or blue) shirt and tie. You will look smart, look the part but more importantly will not look like some divvy who thinks that a flight instructor position is just like an ordinary office gig!
Be yourself, show savvy, dress like an FI.
Tidy.
VFE.