The difference between a mechanical and a fixed blade is the mechanical does not wind plane as bad and flies much more like a field point. Hence more forgiving. You could shoot field points with 6 or 7% foc and do it accurately with enough fletching. I doubt you would have that much luck with a fixed blade that will catch the wind as it flies thru the air. So if you shot mechanicals you could get away with less FOC like a target type arrow and still have it be fairly forgiving. This is why many shoot them, they easier to tune. It's the wrong reason, but never the less it happens.

More FOC forces your fletchings to work harder. This helps your arrow stabilize faster and stay that way. I have never seen too much FOC make an arrow fly bad. I have seen too much tip weight lower spine and make an arrow fly bad though.

What size are your aluminums? If they are bigger than 19xx then the nocking point will not be the same from arrow to arrow because the aluminums are larger in diameter. Going to a smaller diameter arrow without changing your nocking point or adjusting your rest will lower your impact points.

Also by adding weight to the back of your arrow you could have changed the dynamic spine. Adding weight to the back stiffens your arrow.

Is this a problem you are having with a broad head on your arrow, or with field points? I am confused about that part.

A simpler answer is you can't just change arrows and not anything else and expect things to be the same. Sometimes you get lucky, but usually you need to retune your bow.

Paul