i dont actully know the size of most of your fish in there smillermom.. but if you add up the size of all your fish (in inches) then you will know if it is overstocked or not..i noticed you have 7 cories. these fish stay on the bottom of the tank so they would be sharing the space with the clowns..the rest of your fish wouldnt really get in the way of the clowns as clowns rarely go any higher than half way up the tank and stick mainly to the bottom..you can say bye bye to your mystery snail too if you put clowns in there..
(clown loaches luv to eat snails)..but that could be moved to any 1 of your other tanks easily enough..personaly i think you could get maybe 2 maximum 3 clowns in your 55G..
but i would wait and see what one of the more experienced members say(like gunnie or butterfly)..i know they would get along with all the fish you have listed,as clowns are really friendly and would get along with allmost any fishies..but you dont want to get just 1 clown..clown loaches luv the company of there own kind,and most entertainment will come from the clowns interacting with each other..
..(as well as the other fish)..good luck anyway and if you do get some it would be great if you could post some pics..baby clowns are so cute..
hopfully you will get a reply from somone else that can give you a better idea as to how much more room you have in your tank..hope this helps also.
again good luck
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I realy like the way you have decorated the tanks. I like the fish as well, I'm sure they are realy happy to have such a nice home. 
