The algae would suggest lack of nutrients.
Put it this way, algae shows up when we expierence poor plant growth. So, it's telling you that your plants are missing something. Your confident in your CO2 levels, so it must be nutrients.
Depends what you mean by overdose. Ofcourse we can add more than the plants actually need, that's one of the main principles of EI. We dose a bit more just to be certaint that the plants recieve enough. Even though the nutrients are in excess, we still dont get algae because we're not giving algae the conditions it needs to bloom. The spores are looking for ammonia+light, not NO3 and PO4. There's bound to be a toxicity limit (to fish and shrimp) on what we dose however we havent encountered it yet. You'd have to dose ridiculous amounts of dry salts, (100s and 100s of grams) in a short amount of time to experience adverse efects on the inhabitants. Like I said, none of us hobbyists have encountered deaths due to nutrients. CO2 and liquid carbon is the cause of most deaths in a planted tank.