I wouldn't add another, goldfish are lower in quality than they used to be so a 60 litre biorb isn't an ideal home for them. Goldfish do like company but you'll do They are fine kept alone really. Are they fancy or regular goldfish? If fancy then they probably only have a few years left anyway.
Nah, he'll be OK on his own. Biorbs aren't great for goldfish and then you've got the added issue of quarantine- you are meant to quarantine new fish for 4 weeks before adding them to existing stock. Realistically, a lot of people don't do this. New fish can easily introduce pathogens to an existing tank and with an elderly goldfish you don't want to risk that.
I'd just wait and maybe get a heater and some neons for the biorb if you want to keep it after the goldfish have gone to the big pond in the sky.