Is it okay to wear a fly plaid with a daywear jacket?
No.

Around here fly plaids (and miniatures) with formal wear are so common as to be considered normative. I always wear one when going black tie (I've only gone white tie twice in the past ten years; such events are extremely rare here), but I'm a piper; I'm expected to be fully "tricked out" at formal events.

I generally wear a dirk too. Also somewhat of a PITA, but quite popular for dispatching haggis and wedding cakes.