Seneye Wifi

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.

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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.

A word of caution - don't believe anything a shop says until you have researched for yourself.

Yes, female bettas should be kept alone or in a group of at least four. One is fine. With two, one will bully the other. With three, two will pick on one. With four and over, the aggression gets spread out and no one fish gets picked on all the time. But this is in a tank with just female bettas. If you have four females with a male, the chances are fairly good that the females will gang up on the male.
Few shops seem to realise that it is not a good idea to mix males and females. It can be done in a large, heavily planted tank but it is still risky.
I've even seen a shop with two rather finless males in the same tank 

Keep a close eye on them and separate the male if you see anything untoward happening. One of those nets that hang on the side designed for fry is a good temporary solution but make sure you cover it with something as bettas can jump. If this does happen, the best long term solution would be a separate tank for the male, something in the region of 20 litres or over.


If you get shrimp you'll most likely be giving the bettas an expensive snack, I'm afraid. They'd be safe from the other fish though.

Cories won't eat algae like shrimps, and they do need feeding (sinking pellets) rather than rely on flakes making it to the bottom of the tank before all being eaten, but yes cories would be fine to keep the balance.