Can decarboxylation be done under carbon dioxide?
Because nitrogen is the standard inert gas in lab practice. If you are "rich" You can use argon, but carbon dioxide is seldom used, because it won't work on alkaline solutions. 


  or will the presence of carbon dioxide slow down the reaction


First, what you presumably mean is influencing the equilibrium unfavourably, don't you? You should learn to discern kinetics and equilibrium. 

Answer: not really. Whether you have a carbon dioxide atmosphere or not will shift the decomposition temperature not more than some degrees.