We had our tomcat, Lightbulb (RIP), on a leash for almost 10 years. Then our little girl, Neko. By the time Lightbulb finally figured out how to wiggle out of his harness at will, he was too old to do any harm to anybody.
(He was never much of a fighter anyway - he'd just sit there & argue with any stranger he didn't like. Well, except for the mailman. He did swat a few mailmen in his day ... but he never had a problem with the UPS guys! He was a Libertarian cat, I think. :-)
Anyway, the point is they get used to the harness if you start them off early and give them a long leash, like 20 ft. or so.