I really like my bristlenose pleco - they stay fairly small (4-5" max usually) and are good cleaners (usually). They DO need other food though, don't be fooled by the name "algae eater". They can and will, but they need something else too. I feed mine veggies and he also eats whatever the fish don't eat, including frozen shrimp, bloodworms, pellets, etc. BNs do not have the nasty habit that commons have, which is sucking on the slime coats of other fish. That is number one reason to like them really.
If you get a new algae eater, he should be quarantined in a separate tank just like any other addition. Watch him carefully for a few weeks to see if there are any problems. I had a pleco that seemed fine even after QT, but he just never grew and I found him dead one day for no reason. I mean to say that they can get diseases/parasites just like any other fish.
Unfortunately, you don't really have the space for any other fish in your tank. Your goldies will soon need a bigger living space themselves (at least 30 gallons if they are fancies, more if they are single tails). I would suggest holding off on the algae eater and upping your cleaning regimine instead until you can get a bigger tank. If you can't get a bigger tank, then I don't think the algae eater is a good idea. They need 10 gallons each (bristlenose, and they are the smallest adult plecos you can get), which will be a strain on your cycle. I think at the minimum you would need a 40 gallon tank to support your 3 goldies and a pleco. Just a suggestion, but your fish will be happier and healthier for the extra space and cleaner water.
