Mismatched tartans are perfectly acceptable, including "clashing" colors. While to many modern eyes this glares a bit, I find it rather refreshing.

If you look through contemporary paintings you'll notice that some wear completely contrasting tartans, some may have two garments that match, and some may have an entire outfit (including tartan hose) that matches. Last evening I found a drawing of a Jacobite wearing tartan breeches and hose of different tartans instead of the usual one piece trews. It's giving me some ideas...

Also, unless you can date a tartan to pre-1745, try to get a generic tartan that can't be identified by the tartan spotters.