i don't think you can have to much co2 for plants as they only use what they need but it will kill your fish if the levels are to high. CO2 can have an affect on the algea growth, to much and the algea will start using the extra co2 left by the plants. that goes for all the nutriens in the tank. if you can get the levels to the right point you shouldn't have to much trouble with algea.
As for big plants, my amazon sward has grown to 50hX30W and i know some other swards do it quite nicely, um lotus will grow to the top and looks cool, twisted val nice long plant with slim leaves, plants like stargrass that are bushy but grow fast look awesome as a background plant..... um lost my train of thought now lol but hope this novel has helped.
I,m not a pro but over the last few years getting in to planted tanks thats the basics I've picked up
P.S. ask P44 about plants and Jennifer about CO2 and nutreints+plants