With a single betta I think one will be fine since they don't have much of a bioload. If you get something else it is always good to over filter a bit. Bettas let you slack on filtration if you want though.
Biomedia is a type of media you put in your filter and the only job it has is to house beneficial bacteria. Some are plastic, but those tend to be for sumps/wet dry filters. The other ones are usually ceramic or a type of glass (It doesn't look like it so idk) They are porous so they provide a greater surface area inside the ball than the amount of room it takes up. I used the Eheim one because I had it left over from my canister filter, it is more expensive, so if you want one you could go with Seachem's matrix bio media. I just have it lose in the area behind the two sponges as well.
Oh okay. I see your tank is planted...do you think that filter would get the job done on a non-planted 5.5? And how do bio balls work? Oh, and with the flow turned up all the way do you think it would bother a long finned betta?
I have it on the 5.5, I would rather have two but it gets the job done. I added bioballs since there is a lot of in touched area you can add, nothing real serious, but the Eheim substrat pro worked for me. I would get another because they are only $12 on Amazon, but i may be getting a Zoo Med canister filter for the tank and upgrade to a nicer tank, but it would be a downgrade in terms of the sizing 5.5 to 4 gallons (rimless)