I'm not buying a larger tank for the goldfish. They're only ~3" long with the tail, ~1.5" without. If they get much bigger I'll get a larger tank.
Back to the original question.
I'll look into bristlenoses, thanks.
that's a fun circular argument you're using there. the problem is that if they stay in the small tank, they WON'T grow. they'll just die. as a protective measure, fish release growth-regulating hormones into the water. these hormones, along with nitrates and other dissolved chemicals work against the fish, stunting it. the outside of the fish stops growing as a response to the environmental indicators, but those same chemicals do not have an effect on the inner organs of the fish. the organs continue to grow, which affects their efficency and eventually kills the fish. a stunted fish will live a dramatically shortened lifespan, will have muted colors, an abnormally-shaped body, lowered immune system response, and be very susceptible to both diseases and bad water quality.
as far as your original question, bristlenoses do not belong in 10g tanks. they reach 4-6 inches, which puts too much strain on the biological system if they are kept with anything else. while they eat algae, they also need to actually be fed with algae wafers, shrimp pellets, and fresh veggies. they can not survive on algae alone - there is simply not enough bulk in algae to feed them properly. also, bristlenose plecos, along with pretty much all algae-eating fish, are tropical and should not be kept with goldfish because they do not have the same temperature requirements.
tank with algae -> algae scubber removes algae -> water changes remove detached algae and nitrates (algae food).
tank with algae -> algae-eating fish eats algae -> algae-eating fish poops and respirates (releases nitrates into the water) -> nitrates rise -> more algae grows, feeding off nitrates -> fish eats algae. releases more nitrates-> more algae. . . .