I definitely think one can wear a fly plaid with an Argyll. (or any other kilt jacket/doublet for that matter!) Every time I make myself a kilt I make a matching fly plaid as well. (It's best to do from the same piece of fabric.) I also don't bother pleating the one corner. This way the fly plaid still has some use as a garment - to be taken off and used as a shawl/wrap for me or me ladywife!
As long as your accessories are appropriate it should look fine.

Is mise le meas,

Seán Liosliath Ó hAirt

P.S. Does anyone know what a "day plaid" is? I've heard it described as a folded cloth over the shoulder (wow, how descriptive!) but have never seen a picture.