Possible to over fill trans? 03 grand am

I just recently did a transfluid change on my grandam. I took the cooler line off, started the car, and drained the fluid out of the pan that way so when I drop the pan I wouldmt make a mess. Then I dropped the pan and took the filter off, and I noticed another plug on the front of the trans by the cooler lines, and I took that off, and fluid seeped out, so I strarted it to see what would happen, and fluid started gushing out, so I let it run till it was empty, about 2 mins. Then I put the new filter in, cleaned the sludge on the bottome of the pan, put the new gasket on and reinstalled the pan. I filled the trans with like 9 quarts before the fluid started to come out the side of the transmission. So I put the plug back in, drove it, and it was shifting hard, come home, took the plug back out, and no fluid coming out, I did this twice. Finally after 13 quarts, the trans was full. I put the plug on and added some more, after that just in case, and was curious to know, if you over fill the trans on this, will it blow out if nessary? I thought about taking the plug out and letting it drain to the proper level. Should I do this or what? Thanks, any tips would be helpful, and I want to add that the transmission shifts so much smoother now since i added my Schaeffers All Trans Supreme.