Retiredadguy, I certainly wouldn't say "never." Not recommended, certainly.. but never? I have put enamel over acrylic many times in the past with no issue. As long as the acrylic is dry and you apply the enamel either VERY carefully by hand or via airbrush you shouldn't (read: 98.9% of the time) have issue.. just comes with experience bending the rules I suppose. Learning exactly what the tolerance is of acrylic and enamels. You can also get interesting effects and new techniques mixing these mediums.. oil and water at its finest.
As for future, it is just an acrylic sealer, it isn't a wax. Can be sprayed out of an airbrush right from the bottle, goes on nicely, self levels, and cures crystal clear and strong. It's very common on smaller models, but I use it quite a lot as a sealer over faux-metal and chrome pieces. Most sealers cut the shine of chrome down to a dull silver but Future doesn't which is very handy. It works even better than Alclad's Aquagloss.
Most recently I've gotten into the habit of using it as a barrier coat between the lacquer metal paint and a 2pack (urethane) clear coat with great success.
As for your final point, I agree. Test, test, test! I always have Speedshapes on hand to good around with new paints, techniques, and general tomfoolery.