Yeah, you can over do it with anodes. They will react with the copper in the bottom paint.
Heck--- on my old Hunter 30, I USED NOTHING!!! AND NOTHING WENT WRONG after 25 years of sailing! The best way to determine if an anode is required is to measure the various wetted metal voltages against a half cell.
I now own a Silverton 40' aftcabin. What I found was everybody gave advice about anodes....I gave up on zinc, I now use aluminum anode. The first couple of years I owned the boat I copied placing zincs all over the bottom as did the previous owners. What a waste! I knew there was a better way to solve this problem.
I cruised Ebay for a while looking for a silver-silver chloride half cell and was lucky enough to purchase one for $40!!! What a steal! The thing new was priced over $600 as a part of some test gear to corrosion testing. Anyway, I now use a single aluminum anode plate mounted on the stern and that is it!!! No shaft doughnuts (which never seemed to stay anyway), nothing on the rudders or anywhere else.
I also made my own prop shaft brushes using oil impregnated sintered bronze bearing to bond the shafts. They work although there are better methods to bond the shafts such as something like a wire brush or even a carbon electrode.
After doing those things above, I measured the voltages between each piece of exposed wetted metal and the half cell. I am well within the safe range for corrosion prevention.