The real problem as I see it is that whatever you wear that is "different" will also be, in your words, "costumish"

Fly plaids are not really a good idea at all, with anything, unless it's with a kilt and doublet at a very formal affair. A tartan waistcoat would look decidedly odd with a Tux I think. Maybe a tartan cummerbund but even that might look strange

Why not simply wear the tux and save your tartan statement for the time when you can wear full highland dress?