That depends on the number and size of the fish you have added in the tank. But as blanket recommendation you can do 50 percent water change once in a week.
weekly 10 - 20% water changes
That's perfectly normal for a betta.
It depends on how often you clean the tank. you dont need a sucker fish if you clean your tank alot. If you dont clean your tank that often then get one. you should talk to your local pet store for more information.
you should change it every week so it will always be clean.
The fish are not bad together. But way to many fish for a ten gallon tank. You put one fish for every gallon. They will probly do fine but you will have to clean more.
The rule of thumb is usually a gallon per fish, so six gallons would be good. If the fish are on the large side (like a goldfish), then a seven gallon tank would be ideal.
Betta fish are about the only kind of fish that can survive with no filter. They prefer still water or a very gentle current, so as long as you are keeping their water warm and clean (at least 80* F) they will thrive in a one gallon, unfiltered bowl. Change the water at least weekly to keep a one-gallon betta tank clean. Change more often in anything smaller (and it's not recommended to keep them in anything smaller).
A betta fish would be much more suited to a 1 gallon tank than a 'variety'. You shouldn't keep much more than one fish per gallon anyway.
Once a week
Depends on the kind and size of the fish. You should also take in to consideration how large they will get at adulthood. A good rule of thumb to go by is 1 gallon of water for every 1 inch of fish. So for example: 3 2" fish should be in at least 6 gallons of water. To be safe I would put them in a 10 gallon aquarium.
I personally would empty it, then scrub it clean and use it occasionally as a flower vase. It is not large enough to keep any kind of fish in properly. The basic rules for keeping fish successfully are. :- 1 inch of fish needs at least 1 gallon of water (a Betta is a 3 inch fish). Every tank must have a permanently running 'cycled' filter. Every tank must have at least 50% of its water replaced every week. The above is not completely true. A Betta in a half gallon tank should have half of its water replaced each week. You should use a conditioner with that water (mine came with Nutrafin Betta Bowl Water Conditioner). I also make 20% of it HBH Betta H2O, which can be bought in bottles.
I cleanmine once every week if the light goes out every 4 days