From what I understand, 'gobies' are the family of fish that comprises of zillions...so, there are sand sifters, swimmers, diggers, leapers, whatever...very plentiful. You can't say they wouldn't be good in a 29g because there are so many of them that don't get very big at all.
I believe blennies are acually closely related, too.
And, of course, you have blennies that eat algae and blennies that are really dragonettes and eat pods. So, dragonettes are practically gobies, too. And some dragonettes stay small, while others grow too large for a 29g
Having a small sand burrowing goby in a 29g would probably be fine, whereas a 29g tank with a shallow sandbed and a large goby would probably lead to issues.
Sand sifting starfish, from everything I've read, are not just a risk to their personal health, but they also eat all the stuff that makes a sandbed worth having, so in effect could also be VERY detrimental to the health of the entire reef tank.