Over filling is not good, it was designed to hold just so much fluid...that's all. The level should be checked with the engine and transmission at operating temperature, at idle, in park.
DO NOT flush the transmission. The "flush" is nothing more than a fluid exchange. Duriing the exchange, new fluid is forced through the filter, along with any contaminants that are loosened during the flush, which will clog the filter, and possibly oil passages in the transmission.
The proper way to service the transmission is to bring the engine and transmission to operating temperature by driving approximately 15 miles. Shut the engine off and immediately remove the transmission oil pan and filter while the fluid is still hot. Allow the old fluid to drain out, and replace the filter. It's best to use a lint free towel to clean the oil pan, the mounting surface of the pan, and the transmission case, make sure you keep it clean. Install a new oil pan gasket, (do not use any type of adhesive) and install the oil pan. Be sure to use a torque wrench, and tighten the bolts evenly to the proper spec around the pan. Add approximately 4 quarts of the proper fluid and start the engine. Before checking the fluid level each time, move the shifter slowly between park and low gear. Top off fluid level until full. Check for leaks.
Again, DON'T get a fluid exchange or a flush, you're asking for trouble if you do.