The Aqua CLear is a super filter and the 30 would probably suit your needs as long as you are not intending a single goldie in your 25 gallon, where I would recommend moving up one size...
I have a number of them and they work well. You would add the AC
to your present filter set-up and continue on in the fishless cycle.
If you are lucky you might be able to just run the AC
alone once it has cycled...if not you could run them in tandem for a couple of months and then the AC
should be able to do it by itself. If there is other media--bio balls, etc, besides the charcoal in the 1st filter, you could also add that to the AC
to hold the cycle.