i have a 50 gallon with now 5 goldies in (was 4 until a mum at the school who knows i have fish asked me if i could take in her teeny lil fantail goldfish that was living in a 2 litre tub thing-apparently her hubby brought it for their son-and i couldnt say no) so now im very very over stocked, but i just do 70-80% water changes once a week on each of my tanks, i run 2 filters on the tank, a fluval 4+ and a fluval 3+, plus a big airstone just for that little bit more movement in the water. i also have a 30 gallon with 2 male goldies in, that runs with a fluval 4+ and a fluval 2+ and an airstone. my tanks both have live plants in, just elodea as i dont have high lighting, but the fish seem to love them.
clamped fins i havent had to deal with in the past few years, since i learnt what cycling a tank was and that me taking the whole tank down every week and cleaning in tap water was the number 1 worst thing i could do. since my tanks have been cycled and properly cared for when it comes to dechlorinator and not being lazy and leaving the tank for 'just a few more days' i havent had any problems.
coping with the mess they produce- well having gravel on the bottom hides a lot of it i suppose, but i do do very thorough gravel vacs every week at water change, and it removes so much poop its unreal. i always vac until barely anything can be seen to be removed.
feeding, i feed mine mainly on fancy goldfish sinking mini pellets twice a day, they reduce air intake so the goldies tend not to get problems, but i also at least twice a week feed shelled peas,and also as treats 2-3 times a week either brine shrimp, daphnia, orange segments or blood worm. they always get at least 1 feed a day of pellets. my fish are growing well, no clamped fins etc, and have fantastic colour.