Yes it would be enough, you are not actually cycling the water, just the filter media. But it would not be pathogen free. If you had any flukes or whatever in the 55 they would be transfered to your 29g. To be absolutely sure you would need to do a totally clean fishless cycle with new media, about 5 weeks. Some folks will use Safe Start or other bio in a bottle to speed things up. I find those products useful only if the filtration needs a pick me up after using meds, but the Safe Start is the best one I've tried so far. You can double deck/stack the sponges placing the new larger one on the bottom and smaller one over that. The Hydras come with an extra extender in the box. A few weeks and the new one should be good to go.