To answer your questions:
The carbon cartridge. I would replace it with sponge, or perhaps a layer of filter wool that you replace regularly. The outer layer of the cartridge is probably made of something similar to filter wool, which is designed to trap small particles and gets clogged quickly. Filter wool is not a very good medium for bacteria to grow on and if it is replaced regularly it won't grow many bacteria anyway so throwing it away will do not harm.
And you don't actually need to run carbon full time.
You don't need to replace ceramic media until it crumbles. The instructions will tell you to replace it every so many months - this is a money making ploy for the manufacturer. When the ceramics to start to disintegrate, depending on what else is in the filter: if the media is nearly all ceramic, replace a quarter every month till it's all changed; if there are other things in there as well, replace a third to a half every month.
Seneye - not really into gadgets so don't know
Bottom dwellers - I have pygmy cories in my 50 litre tank. In your tank you could fit a shoal of the larger panda cories. But cories do prefer sand rather than gravel, though they might be OK if your gravel is smooth.
Attachments - click on the 'attachments and other options' box below the reply box then use browse to find and click on your photo
Your stocking - how long have you had that combination of fish? Because you have some combinations that are not recommended.
Male bettas often mistake male guppies for other male bettas and attack them
Neon tetras have been known to nip the fins of male bettas
Male and female bettas should not be kept together except when breeding them, and then only until they have spawned and keeping a close eye on them for aggression. One or other usually ends up badly beaten up during spawning.