For rabbits, squirrels, and fox, I don't think you could do much better than a 20 guage shotgun, providing the shots at the fox would be close (less than 30 yards). If not, then I would go to a .22 LR. You said you like cowboy style guns. In this case, Marlin makes a lever action .22. If I were to go with the shotgun, I would look into H&R's 20 guage single shot.

Marlin Model 39A ... http://www.marlinfirearms.com/firear.../Golden39A.htm

And the NEF (H&R) ... http://www.hr1871.com/firearms/index.php?cat=3&subcat=1

BTW I have a Marlin Model 60 and have not had any problems with it, nor have I had any trouble with my H&R .223!