So, I'd like to address a couple of challenges first and then make a suggestion that might work.
Size of Tank
The ponds you currently have are basically sized for a single goldfish. The risk factor here is that more than one goldfish will overload the environment as their waste product can be toxic to them.
Goldfish are very hardy and can survive in less than ideal conditions, but it's not necessarily true that they're thriving. Bear in mind that a healthy goldfish should reach to about 24cm by their second year. Look for black spots or abnormalities on the fish to detect the effects of their waste product on them.
Now, the RSPCA of Australia lists that 50 litres is a minimum tank size. So, there is a little bit of dispute about the minimum and if the fish is healthy, you may have some room to spare.
Also, as you grow the size of the environment, the rule of thumbs are best to keep in mind. Ideally 20-30 gallons to start, add 10-15 for each new one. Err to the side of more, it will never hurt to have more space.
Goldfish are omnivores that eat plants and other aquatic animals and they're opportunistic feeders. They will, of course, eat other fish as consequence, especially if the fish can fit in its mouth. So, obviously, any companion fish for them needs to be bigger than what the golden fellow can directly eat otherwise they may dine on your intended companion fish, including other goldfish.
Getting additional goldfish may result in having more than you expect... Unless you've correctly sexed the fish. Something to keep in mind. Mind you, domestic goldfish will often eat their own eggs, so you may not end up with a litter as it were anyways.
So, goldfish are social creatures and that does argue for your concern about companionship. I don't think your tank sizes are ideal for having an additional goldfish, I think you're pushing the edge of good size for a single. The link I just gave makes some recommendations on alternate companion fish. Of those, I would actually recommend going with the danio (or zebrafish) as being the most suitable for your conditions.
The danio are hardy fish and social fish that get along well with other species. They're active and playful, as are comets, so would seem to be a good blend. They're easy to keep and feed being omnivores. Tank size recommendations are about 5 gallons per fish, so you might be able to get a couple in there. I would probably start with one and see how it gets on before adding any additional ones.
I realize that I've pushed on the tank size a bit and I have because hardy fish species can create an illusion of thriving when they really aren't. While I agree that your goldfish need companions, I think you might want to pay extra attention and care to their health and activity if you introduce additional fish to their current ponds.
So, adding the danio today isn't a bad idea and may alleviate the initial concerns about the the current fish. With respect to having more goldfish together, given a future environment, the considerations are:
Size. Larger fish won't normally be an issue if there is sufficient space, but if a larger fish is much larger and more aggressive, it may be best to move them out (last paragraph) or separate from the smaller fish regardless of space.
Type. Mixing varieties, especially common with fancy, can lead to problems. The comet variety, as I noted earlier, is an active fish and so you probably want to keep like to like.
I would generally try to size the new fish to be reasonably close to the size of the current ones where possible. This is not an absolute must, but will enhance the chance of the introduction being successful. Bear in mind that you can still keep the danio if you do end up adding them to your current ponds.