Seems like you have the right idea. I too don't use cartridges for the cost and because they are expensive, and the activated carbon leaches valuable nutrients from the water column that your plants could use. Load up all of those empty spaces with ceramic or other media. The only thing that I would say is that whatever your main mechanical filtration is, you ought to rinse it out using tank water during WC so as to minimize the disruption to you BB.
3.5 gallon- Dwarf Cories
10 gallon- Cories
5.5- Khuli Loaches and Golden Tetras