could they wear a Cunningham tartan
Mate, as a Cunningham, people have no idea what the tartan is, and have even had comments like "is that a MacDonald?" and other various types of tartans.

My old lady (mother) is/was a Mcleod, and when i was getting my kilt built, quite frankly stated " I hope its not that ghastly (changed terminology) red one"...

Note that the Ayrshire tartan may be worn by anyone with connections to the districts of Cunningham, Kyle and Inverclyde.
Bet an educated Montgomery wouldnt wear it...