New Filter / Running 2 Filters - New .v. Mature Media Placement, Etc.

I've set up my new filter - a JBL Cristal Profi internal i80. It's a corner filter and is nice and streamlined and quite hidden, blending in well with the background - exactly what I had hoped for. It's completely silent - a marked contrast to the one which had been getting louder and louder as the week wore on (the manufacturer said it was likely the motor was failing) and which had reduced to a mere trickle of outflow by this morning. The packaging/instruction booklet recommended the wide-shaped nozzle as the outflow for a planted tank so that is the one I have it set at.

The filter comes with 2 triangular compartments, each with a thick round sponge with a hole in the centre for the central tube to run up through. There isn't enough space to squash sufficient existing/mature filter media into it around the edges and I wasn't keen on cutting the new thick round sponge up eg so that each compartment had 50% new media and 50% mature media, as I'm hoping that ultimately it will contain just the new media once matured. Therefore, what I have done is:
* removed the thick round sponge from the bottom compartment and filled it entirely with existing filter media (about 80-90% of the existing filter media has been squashed in here);
* left the new thick round sponge in the upper compartment, squashing some thin mature filter media (about 5-10% of it) around the edges [apparently the water flows from the bottom up, so I thought this was the best place for it as it ought to "wash" the beneficial bacteria upwards];
* set up my spare Fluval U2 filter (a monstrosity in a tank this size) as well, as it was the only filter that could accommodate the new thick round sponge from the bottom compartment, and I've squashed it plus the remaining 5-10% of mature filter media squashed down the central hole into its large central compartment (having removed the several vertical trays from its inside).

I'm hoping that, after running the two filters side by side for about a month, that the new media in the U2 will be sufficiently mature to replace the existing mature media in the bottom compartment of the Cristal Profi, and that the numerous pieces of falling-apart existing mature filter media can be discarded (or perhaps put back in the mini filter which I've been running as a second, back-up filter in recent years and might / might not do again, once the U2 has been removed).

The alternative would have been to run a fishless cycle on the new media in the U2, using a QT to do so, rather than having the U2 simultaneously in the tank for a month, but I didn't think the "unnecessary" set-up of the spare/QT would go down too well with Mr FCMF.

Grateful if I could have confirmation/advice on:
* whether this sounds ok or is there a better way of doing this?;
* whether I need to fill the large (a couple of inches) gap above the media in the U2? I'm assuming that, if I did so, this would pose a problem in a month's time as there would be a larger percentage of media being discarded than may be being discarded. [Edited: afterthought - might it be worth adding the 2 longstanding moss balls from the aquarium into the filter to at least partially fill this gap, in some way acting as filter media?]
* If I didn't run the mini filter as a second filter, I'd be discarding a lot of mature media - what is the likelihood of discarding that quantity on incurring a mini-cycle? I'm aware that it's usually advised that up to a 1/3 of media can be safely discarded but it's quite difficult to quantify the exact amount given that some sponge is thick and coarse while other sponge is finer.
Many thanks in advance.

Thanks, Matt - that's very helpful. On the basis of past experience, I generally tend to adopt a "belt and braces" approach. However, if I were to remove the U2 altogether, would there be any merit in leaving the-new-sponge-with-old/mature-sponge-in-its-centre in the tank before transferring it into the filter in a few weeks' time? I could probably weight it down with the pieces of Tufa rock.

Great; that was my thinking - and good point about putting it near the flow. Thanks.