IIRC stands for "If I recall"
Any type of mature media will have a chance of working. In most cases its probably the 80/20 rule, 80% chance the bacteria in the introduced media will live and help your Fish-In cycle (you're doing Fish-In, I see it now I think...) to go faster. The reason sponge is mentioned is because floss media will break down sooner and might not last quite as long as sponge would, but its hard to tell whether this would make much difference in a donation case since you mostly just want it for the innoculation effect anyway.
In cycling, we're trying to grow two specific species of chemolithoautotrophic bacteria which carry out the Nitrogen Cycle that one learns about in environmental science. These bacteria cling to the biomedia surfaces and flourish inside the filter because fresh oxygenated water and ammonia (their food) is being pumped through in volume. The biological filter is such an important basic of the hobby (and indeed of the wider field of environmental water management) that the particulars of which media materials present the most desirable surfaces to these particular bacteria have been refined and refined some more. Currently sponges and ceramic media are about at the top of the choices for ideal biomedia materials. Thus the implied preference for sponge. But in your case, for simple seeding of the fishless cycling process, it shouldn't matter much. And I think "between the sponges" sounds like a great idea if you can make the fit all work out.
You've probably already read it, but the ammonia you will introduce will be processed by the first bacterial species (we like to call these the "A-Bacs", short for ammonia oxidizing bacteria) and for each 1ppm of ammonia, they will produce about 2.7ppm of nitrite(NO2) which eventually will be processed by the second species of bacteria (we like to call these the "N-Bacs", short for nitrite oxidizing bacteria.) The N-Bacs will eat the nitrite(NO2) and produce even more of their product, nitrate(NO3), which will eventually be removed, along with other trace impurities from the aquarium via a (weekly, ideally for beginners) water change.
When performing a Fish-In cycle, one must go about the procedure, getting help from the members here (as not all details are ever necessarily clear to anyone coming in on it new, even given our good guideline articles) regardless of whether there is mature media or not. Your main goal will always remain as keeping ammonia and nitrite as close to zero as possible and working out a shedule and percentage water change sizes such that neither bad substance has a chance to rise above 0.25ppm. Normally this takes about a month but if the mature media takes hold, it could drop to 3 weeks or even less.
~~waterdrop~~
