There definitely isn't a rule against entering a pet shop ham in main class -- but as kyrill said, a pet shop ham may not do so well! My old Syrian had the dubious honour of being bottom of the show table the one time I entered her in main class
Plus for main class it's more "hassle", as you have to hire (or buy) specific show pens, be there earlier and stay later than you'd need to for pet class, etc. Not that I am saying don't show in main class, but that pet class is (I believe) the best place to start.