It is wonderful that you have been able to keep several species alive for over 10 years!
I foresee problems with the current stocking.
Golds seem to produce more ammonia than other fish of similar size, so under stocking them is better. 25 gallons minimum per fish. The often cited idea of "20 gallons for the first and 10 gallons each after that" is nowhere near enough for full sized Goldfish.
Loaches in general are a bit more sensitive to poor conditions, so should be understocked because of that.
An over crowded tank is simply a disaster waiting to happen. If there is any problem at all (power out, or anything) the tank will crash (conditions become toxic very fast) and you are likely to lose fish.
More fish mean more carbon dioxide in the water, and less oxygen, and more wastes of all sorts, most obvious as nitrate. If there are too many fish, and not enough locations for nitrifying bacteria then ammonia and nitrite will start showing up.
Fish produce waste and need oxygen based on their mass, easiest to think about as length x width x height. A fish that is twice as long as another similar shaped fish will have twice the width and twice the height as well, and therefor need 2 x 2 x 2 times the volume of water to supply oxygen and to dilute CO2 and other wastes.
Another issue with adding more fish is the social aspect. Even highly social fish need more room, and Loaches are certainly social, but can also get quite pushy or even aggressive with each other. They need their space.
In a 55 gallon tank I would put a maximum of about 6-8 fish at 4" long, if they are of average activity levels and reasonably peaceful.
I would not put any fish that grow to a foot long in such a small tank. Maybe 6" max, if it is a sedentary fish.
Goldfish can reach a foot long or there are the shorter and fatter fancies, but either way they need a lot of water to dilute their waste between water changes. They can get pretty active, so room to move around is important. Especially the Comet or wild shaped fish. A tank that is only 4 times their length is not enough running room. They are usually peaceful with each other, so volume for social reasons is not quite so important.
Clown Loaches can also reach a foot long, and are quite bottom oriented, appreciating caves, driftwood and similar features. To have enough caves etc. for half a dozen foot long Loaches requires a large tank. It is the simple physical fact that they need the space of a much larger tank. They can get mad at each other, and there will be times when each Loach may want to retire to his own cave. They can be even more active than Golds, and seem to coordinate their activity so all (or most) of the Loaches are playing around at the same time.
http://www.loaches.com/species-index/ya ... ia-modesta
Read the lower half of this page, "Notes"
No, I do not see it as a good idea to keep Y. modesta in a 55 gallon tank at all, and certainly not if the tank is already overstocked with other bottom fish. There simply is not enough room in that tank.
It is a good idea to attempt to get more of the same species, but upgrade the tank, too.