When I did something like this a few weeks back for a new (to me) 29 gallon I did the following:
1. I took some gravel from 3 other existing tanks and mixed it with new sand for the substrate. Probably not the best for maintaining bacteria, but I wanted a mostly sand substrate in this tank because of the species I was considering stocking it with at the time.
2. I filled up the new tank 50% with water from the other tanks and 50% new tap water.
3. I used a filter bag and media from an existing goldfish tank.
4. I moved 2 ornaments over from existing tanks as well as adding a couple of new ones. Plants from existing tanks were also moved over.
5. I put 2 mollies and a 7" pleco in the tank after 24hrs.
The tank took about 2 weeks to cycle. Ammonia levels went down quickly and nitrites took a while longer. I had cloudy conditions for about 2 days at the start but the water has been clear since and testing at 0 ammonia and 0 nitrite for about 2 weeks. However, I had a nitrite spike (~.25-.50/ppm) and slightly cloudy water this week (4 weeks out) after a 3 new fish were added. Daily 10-25% water changes seem to have brought it back down and last night's reading showed 0/ppm again...no fish losses.
So, my experience has been that although the tank may appear cycled quicker by this method, it may not be 100% stable and you'll need to monitor it closely, especially when adding new fish. I guess the other lesson is to be more patient and wait a full 6-8 weeks before stocking beyond the cycling fish but we know how hard that is!