As Ron referred to, when wearing a tartan that you're not affiliated with, you need to be prepared to justify it. Somebody at some time will recognize that particular tartan and call you out. Wear it proudly and with respect and I doubt that others of that clan will object. Prepare to acknowledge why you wear it and perhaps even understand the occasional annoyance of someone in that clan.

More than clan affiliation, I can understand why military members might object to a non-military guy wearing Army, Navy, or Marine tartan.