I personally would get a reptile... a snake or lizard maybe, but you can put almost anything in a tank; fish, small animals like hamsters etc if there are breathing holes.
either a 15 gallon tank or a 20 gallon tank
No you should not 2 bearded dragons should be put in at least a 75 gallon tank. You could put them in a 55 gallon tank but they would be extremely stressed out.
About a 20 gallon tank.
NO
Only 1. Usually a Betta is all that is kept in a 3 gallon tank
no you should not
2-3 small hermies should go in a ten gallon tank but you should at least have a ten gallon tank for 1 hermie, you should think about getting some friends for it too.
Some say they need a 5 gallon tank, but a 1 gallon tank should be ok if they are by theirselves.
The tank should be changed partially (25%) weekly.
It should be a ten gallon tank and should have a bowl of water to lay the eggs in.
u can use a aqueon 55 gallon filter
A 1 gallon tank could have one betta fish, but you will need to be diligent about changing the water on a regular basis. The smallest tank that should be used, even for a betta, is a 2.5 gallon tank as it at least gives the fish swimming room.