Ok.
The external strip thermometers are not very accurate for telling the water temperature inside the tank. They are great for a quick glance to make sure things haven't went really bad but otherwise you really need an internal thermometer. A glass one with a suction cup tend to be fairly cheap and work nicely. If the heater is set to 24C and the water is actually closer to that temperature than the 26C, then that could be a small part of the problem since 24C is a bit cool for a betta. 25C-27C is the preferred water temperature for the betta to stay healthiest.
In the tank size you have, it might be best to keep the betta by himself. If he has already been known to kill other fish then he probably will be aggressive to the ones you recently added or be stressed out by them. The shark-like sucker fish most likely is not suited for a tank this small. And depending on which kind of fish it is, could be contributing to high ammonia levels in the tank since they tend to be very messy fish.
Flakes and freeze dried foods (what the cubes you recently bought sound like) are not the best for bettas as they contribute to overfeeding, bloating, and constipation. As well as potential to foul the tank with uneaten food -- cause a "pinch" is hard to measure the true amount you are feeding. I recommend a pellet food made specifically for bettas with the first few ingredients not being "fish meal" or any other time of grain product. Supplementing with frozen or live foods are other nice options for you.
The lethargy, hanging out at the surface, and going pale are not what I what call "normal" behavior especially if he has not been this way the whole time you have had him. The fact that he is so bloated could be part of the issue but doesn't really explain the paleness.
As far as your last question(s) -- with the stocking you currently have I recommend at the least 50% once a week but depending on the results of the water test from the fish store (make sure they tell you the actual results not just "everything is fine" ect) you might have to do water changes more often. What might also be part of your problem is that you are completely cleaning and replacing filter pads/sponges at least once a month. If the tank was cycled, then doing this will kill your cycle since the majority of your bacteria live in the filter media. It is just best to rinse the filter media/sponges in the discarded tank water and then put them back into the filter. That works nicely until the media/sponge is literally falling apart -- at which time you replace that piece -- just not all the media at one time.
A picture of your "shark-like" sucker fish would be a good idea as well so that it can be identified.
