Remember everyone community rules state you can give opinions and challenge them but do so civilly and respectfully.
I have been in the hobby since the 70's and have had ponds for 15 or more years currently with a 4000g koi pond and smaller GF pond. I can honestly say that keeping goldfish with tropical's is generally frowned upon unless you are keeping cool water fish such as some barbs, WCMM's, or Hillstream loaches with goldfish.
OP at one time I kept a water barrel with 2 GF and plants for my children in the early 80's. The fish couldn't be kept outside in it during the winter and I didn't want to add these fancy GF to either pond so I brought them in during the winter. I had a large Tupperware tub and put a filter on it. Back then I used a large size Whisper HOB filter. I fed the fish lightly and did WC's weekly. It worked very well and come spring when temps were suitable they went back to their pond. This can be done quite easily.
Now back to the GF with Tropicals issue... Temperatures are an issue because while GF can live in and tolerate slightly warm temps it is a fact it will shorten their life span. Some tropical fish such as some barbs, hillstream loaches, platies, and a few others will do fine in temps in the low 70's to higher 60's most tropical fish do not do well. Then look at the fact that GF produce copious amounts of waste and ammonia which uses up buffers in the water quickly causing ph to be low. This is why most GF keepers do large weekly or even bi-weekly water changes. This is a poor environment for all tropical fish. GF also should be fed with sinking foods, especially heavy bodied fancy GF so they do not ingest too much air which can lead to swim bladder issues. Comets and regular GF are very fast and will often outcompete other types of fish when it comes to eating. These are just a few reasons why they shouldn't be kept together.
Now before anyone's feathers get ruffled I am not saying they can't be kept together BUT just because in some instances it can work doesn't mean it should be done. This can be very misleading to people who are inexperienced or new to the hobby.
I also won't start in on the no WC issue either other than to say WC's are done to remove excess nutrients and toxins that build up in an aquarium over time. They also add buffers to the tank, unless you have a very soft water supply which in that case minerals need to be added, to keep Gh, Kh, and Ph at suitable levels. Tanks left without WC's for long periods of time also use all buffers up in the water (kh) especially in GF tanks which makes the ph drop very low. A ph of below 6.5 causes biological activity of slow and a ph of 6 or below biological activity stops and BB begins to die off. I just want to put this info out for anyone new to the hobby and let's not start a discussion on WC's in this thread.
So OP were it me I would simply get a suitable large Tupperware tub and winter them in that. Depending on the size of the GF will determine the size of the tub needed.