I'll bypass the back and forth and give you my opinion here.
As far as being overstocked bio-load wise, I think you are fine. But you are close to the load that a 55 can handle and I agree with the others that said one partial water change every 2-3 weeks is not enough. You are feeding over 40 fish and that means over 40 fish are releasing waste. Are you testing the water? Or are you assuming the water is fine because nothing has died yet? Honest question there, and if you are testing are you using a test kit or strips?
Now on the issue of social interaction / stocking I see a few problems.
As others have said that poor cory is lonely, they are more social in my experience than tetra and other shoaling fish. In a 55 I think a minimum of five would be good, more would be better. I am afraid of the SDs in the mix, I know you like them and they aren't a problem now, but you should be proactive on this. There are some nice FW fish that you could replace these with, such as bleeding heart tetra or rainbowfish. Just some quick thoughts. Also, the pleco will need a larger tank down the road even more so than the SDs. Most plecos get to be 18 inch at least. I admit they grow slowly but once again being proactive is better than waiting till the situation gets bad. Remeber these are living creatures same as a dog or cat and they are in your care. Respect they health and happiness.
. Anyway If the SD's go wrong, ah, I'll have no choice but might replace them with the fish you recommended then.
