You know how there are different breeds of dogs and if you live in a big house with a big yard you can get a labrador or a great dane and if you live in an apartment you can get a yorkie or something else rat-sized? Fancy goldfish come in breeds too. Some are xbox huge and some are not. I have 5 goldfish that I didnt lose when my yard got wrecked. Biggest is a 9inch oranda (think swimming grapefruit), smallest is a 6yr old 2.5inch short tailed ryukin. She's not stunted, been with me since birth, always had great care and good food. She's just a little fish.
As far as keeping them in groups or not, it depends on the fish. Some dont like being alone, some dont seem to care. They're all dirty fish tho so if you're not sure then dont go that route. They aren't the only fish in the sea lol.
I would reccomend heavy filtration no matter what you chose simply because 29gallons is a tank on the smaller size and it is easier to manitain a smaller tank when it is over filtered.
