My personal view is that mixing Tartans looks somewhat unsightly and as far as mixing Clans goes I would also say that this is A No No!
When I first got serious about kilt wearing I bought Black Watch Tartan flashes to match my Black Watch kilt,after more research I bought original Military Surplus Black Watch flashes (Red Wool) and I now wear them all the time when I wear brogues.
I find that they go well with everything.