In one minor defense of ye olde wikipedia, you can often follow the footnotes provided in the article to the source material. I've often been surprised to find how valid some of the sources are. Granted some articles are bunk, but I think that a large number of them are better researched than many are willing to give credit.

Now to swing back to the subject. Entirely my opinion, I would say that my first advice would be to keep it simple by wearing one tartan in one shading. If you are adventurous, and one of those people who just seem to be able to pull things off, then I would try mixing different shading (M,A,W) of the same tartan.

If you really want to wear a combination of tartans, then once again my personal opinion would be, wear your clan tartan as the kilt. Then if you represent a particular sept, district, or county you could wear that as the plaid. However, I would discourage against wearing just any old combination. Personally I feel like it be interesting to have a plaid that further details your connection to the clan kilt at your waist.

My mother's family are border Youngs, a sept of Douglas. Within my little personal theory I might wear a Douglas kilt and a Young plaid.

On my father's side I read an academic paper written by some yank with a lot of freetime that swears up and down that American Shanklands/Shanklins (I'm the latter) can trace their roots all the way back to the MacGregors. So, having no particular tartan for the sept, I might be inclined to wear a MacGregor kilt and then a Lanarkshire or Glasgow plaid to show the location of the families origin prior to pond hopping.

Pardon my wordiness as this has been mostly an exercise in theory and whim. The short of the long is, if you really want to mix it up I'm not going to yell you down for it, but I would advise to do so under at least the pretense of genealogical logic. Mixing Campbell and MacDonald just because you think they look good, or because you can claim both, is certainly ill-advised.