just wondering when it comes to episodic game do the number of units sold accumulate so they meet the quota collectively or does each
episode need to reach the quota independently before the developer gets paid for that episode?

also as i lack the knowledge can downloadable content be purchased or is it always free?
if its purchased is there a number of required units sold before developers get paid?

if the game is under 16mb but with downloadable contents it become over 16mb does this create any issue with required units or the developer/publisher agreement?

I'm sure the episodes are assessed individually. There's no doubt in my mind. They're sold and rated as separate releases, after all. Combining all of the Homestar Runner games into one big sales total would make just as much sense as combining the sales data from every Megaman game.

DLC is sometimes free, but usually paid-for. It's up to the developer.