Realize that the bacteria on the sponge that you have placed in the "empty" tank will need to continue to "feed" on ammonia and the resulting nitrite to remain alive and growing. Because of this, you will need to make sure that they have a source of ammonia. You can do this with a few drops of pure ammonia, or you can do this with a fish. Since, supposedly your sponge contains nice colonies of BOTH types of bacteria, there is no reason to believe that that new filter is not capable of processing fish waste perfectly - just not in large quantities, yet.
Since you have seeded the filter well ( I am assuming that sponge comes from a well cycled filter), I think that it would be ideal to place a goldie into the new tank. He would feed the bacteria and promote the colonization of the new filter.
Try putting one of your strongest, healthiest little guys in the new tank for a day. Take readings before, and after a day and see where the ammonia and nitrite levels have gone. They may just remain at zero. That new colony of cycling bacteria may be fine to take care of one fishie's waste. Give it a few days and add another fish. Keep an eye on the parameters. My bet is that as soon as it settles with one fish for a few days, with the addition of the second fish, the bacteria will bloom, haze out your tank with new colonies and slam into a full cycle very quickly, all things being ideal.
Just test the tank - at least every day if not morning and night - for a bit until you see the cycle functioning with the single fish's waste.
Sounds like you are doing GREAT!!!!!! Congratulations on your new tank. It sounds pretty!