The Marina 54 is the same tank as I have. I also have the Marina i110 filter but added in an old, similar-sized filter that I had as well - the Marina i110 has a waterfall effect but didn't circulate the water surface much, whereas the addition of the second filter which provides good surface movement means that the two complement one another. It also means that one of the two filters can easily be removed in the event of an emergency / need to set up a quarantine tank without any faffing and swapping of media in what is often a stressful situation. Obviously, it does take up tank space but, because both are quite small, neither actually takes that much space away from the fish - in any case, my containers of limestone rock actually sit underneath each which serves the additional purpose of keeping the filters in place and stopping them from sliding down.
As for space for cables, there are 4 slots - two on each side of the back of the lid. The heater, the filter and the light use up 3 of these 4 slots, leaving one spare for another cable. It wouldn't be enough space for anything larger than a standard cable, though, otherwise it would be at risk of squashing it unless a hole were drilled.
In the i110, I've replaced the existing cartridges with sponge media. It was causing a problem when I was needing to medicate and so needing to remove the existing cartridge to get rid of the carbon and effectively doing a mini-cycle, as also would have happened any time the cartridge would been needed to be changed if it was on the verge of disintegrating. Where the filter wool attached to the plastic surrounds of the cartridge, I cut as closely to the joins as possible and so retaining as much of the filter wool as possible (quite a tough job so sharp scissors were required), then scraped off all the zeolite and carbon pieces from the filter wool, and wrapped this filter wool around some new sponge, before inserting this into the slot where the original cartridge had been. After several weeks/months, I eventually discarded the filter wool as the sponge itself was well impregnated with bacteria from the filter wool. Therefore, both filters have sponge media, and either can be removed in the event of needing to use one in a quarantine tank in an emergency.