I would not add any more fish in your tank. You have already combined the wrong fish around. Goldfish are coldwater/temperate species and are susceptible to fin nipping tendencies which your tiger barbs will likely perpetrate no doubt.
Aside from that, common plecos are the wrong choice of tankmates for 55g. They tend to be more aggressive as they mature and have been known to attack several goldfish particularly the fancy types. The only pleco I would combine with goldfish are bristlenose plecos. They work very well although there are always some exceptions (though rare) which includes sucking of slime coats leaving the goldfish prone to skin infections.
Assuming your goldfish are fancy types, I would rehome your barbs, danios and pleco. Fancy goldfish should be allocated a space of 10-15g per fish. They tend to grow rather huge and bulky. If you have not heard of this yet, telescopes, orandas, ryukins, ranchus and lionheads are a few of the largest variants you will ever find growing to almost 12 inches. I have an oranda at 8" in body length not including its tail. A 55g fits only at least 3-4 fancy types.
The pond types include jikin, watonai, wakin, hibuna (common goldfish), shubunkin and comet. All six grow much larger than the fancy types at 12-18 inches. They are feistier and a 55g cannot absolutely house a single one long term unless you plan to upgrade the tank or relocate them in the pond assuming your goldfish are the pond types.
Your rainbow koi should not be considered in tanks. They can reach 24-36 inches with 18-24 inches being the average size. They are more of pond fish rather than aquarium fish and with the bioload they contribute, tanks cannot accommodate them for a long period of time. They are also far too competitive for the fancy goldfish with the exception of pond types.
As for your tiger barbs. they most likely contracted columnaris. Treat them with Maracyn and Maracyn 2. Your other alternative is tetracycline if your pH does not exceed 7.6 otherwise it will not work effectively in very high pH.
